What is the difference between Unknown Caller and No Caller Id?

Unknown Caller means your phone carrier could not identify the source number, while No Caller Id is a deliberate privacy feature the caller enables to hide their number from your display.

Is No Caller Id the same as a blocked number?

No, No Caller Id is a voluntary setting the caller activates to hide their number, whereas a blocked number is one you have manually added to your device's block list to prevent calls.

Which is safer to answer, Unknown Caller or No Caller Id?

Neither is inherently safe, but No Caller Id often indicates a legitimate caller using privacy features, while Unknown Caller frequently signals technical failures or potential spam networks, so screen both carefully.

Does answering a No Caller Id call cost me extra money?

No, answering a No Caller Id call uses your standard plan minutes and does not incur additional charges, although international or roaming rates may apply depending on your carrier.

Can I switch my phone to automatically reject Unknown Caller calls?

Yes, you can switch on Silence Unknown Callers in iPhone settings or enable Caller ID and Spam Protection on Android to automatically send these calls to voicemail.

Why does my phone show Unknown Caller instead of No Caller Id?

Your phone shows Unknown Caller when the network fails to transmit caller information due to technical issues, whereas No Caller Id appears when the caller intentionally withholds their number.

Are Unknown Caller calls more likely to be spam or scams?

Yes, Unknown Caller calls have a higher likelihood of being spam or scams because robocallers often use unregistered or invalid numbers that carriers cannot identify.

Can I use No Caller Id to hide my number when calling someone?

Yes, you can use No Caller Id by dialing *67 before the number on most US carriers or by changing your phone's outgoing call settings to hide your caller ID permanently.

Is Unknown Caller the same on iPhone and Android devices?

No, the labels differ because iPhone displays Unknown Caller for unidentifiable numbers, while Android typically shows Unknown or a blank field depending on the manufacturer's software version.

What should I do if I accidentally answer an Unknown Caller call?

You should immediately hang up, avoid sharing personal information, and then block the number in your recent calls list to prevent future contact from that source.

Difference Between

Difference Between Unknown Caller and No Caller Id

Written byNex Virox Team

Editorial Team

Reviewed byVarshal Nirbhavane

Senior SEO & Organic Growth Professional · 5+ years

Published: August 30, 202620 min read

Quick answer

The main difference between Unknown Caller and No Caller Id is that Unknown Caller typically means a call from outside your network or an international number, while No Caller Id means the caller deliberately hid their number. Unknown Caller is a network-level label for unidentifiable numbers, while No Caller Id is a user-activated privacy feature.

Key takeaways

Core distinction: Unknown Caller means your carrier lacks caller ID info, while No Caller Id means the caller deliberately hides their number.
How each works: Unknown Caller appears when networks fail to transmit data, whereas No Caller Id results from the caller dialing *67 before calling.
Cost and effort: Unknown Caller requires no action from you, but No Caller Id demands the caller actively enable blocking on every call.
Best-fit use case: Unknown Caller typically signals robocalls or outdated systems, while No Caller Id often comes from private individuals or businesses.
Common decision mistake: Most people assume both are spam, yet No Caller Id frequently hides legitimate calls from doctors, banks, or recruiters.

Table of Contents18 sections

Difference Between Unknown Caller and No Caller Id: Comparison Table

Aspect	Unknown Caller	No Caller Id
Definition	Displays when a carrier or network cannot identify the incoming phone number.	Displays when the caller deliberately hides their number from the recipient.
Purpose	Represents a technical failure to transmit the caller's number across the network.	Represents an intentional privacy choice made by the person placing the call.
Core Mechanism	Network signalling lacks the calling party number, so the handset shows a generic label.	Caller dials a prefix like *67 or enables a per-line privacy setting before calling.
Caller Intent	No deliberate action by the caller; the network simply fails to deliver the number.	Deliberate action by the caller to suppress their number from the recipient's screen.
Display Text	Typically shows "Unknown Caller" or "Unknown" on most smartphone screens.	Typically shows "No Caller ID" or "Private Number" on most smartphone screens.
Network Role	Occurs when the originating carrier or a gateway strips or omits the number.	Occurs when the carrier honours the subscriber's block request and withholds the number.
Technical Cause	Often results from VoIP calls, international relays, or outdated switch equipment.	Often results from a vertical service code prefix or a carrier-level privacy flag.
User Control	Recipient has no way to know the number because it never reached their device.	Recipient sees the label but the number itself is withheld by the carrier.
Traceability	Number is absent from the network signal, making call return impossible.	Number is hidden from display but often still present in carrier billing records.
Call Return	Returning the call is impossible because no number exists to dial back.	Returning the call is impossible from the screen, but *69 may reveal it on some networks.
Blocking Method	Cannot be blocked selectively because the number is never shown to filter on.	Can be blocked via carrier settings, third-party apps, or phone-level filters.
Legal Status	Not regulated; it is a neutral technical outcome of call routing.	Legal in most regions, but some jurisdictions restrict it for business or emergency lines.
Scam Usage	Rarely used by scammers because the number is not intentionally hidden.	Commonly used by scammers to avoid identification and call-blocking lists.
Legitimate Use	Occurs naturally with some international or machine-generated calls.	Used by doctors, therapists, journalists, and domestic-abuse survivors for privacy.
Carrier Handling	Carrier passes the call through without injecting a number into the signal.	Carrier actively strips the number from the display while routing the call.
Smartphone Behaviour	iOS and Android both display the generic "Unknown" label without options.	iOS and Android both display the generic "No Caller ID" label without options.
Caller ID Apps	Apps cannot identify the caller because no number is transmitted to match.	Apps cannot identify the caller because the number is withheld before display.
Frequency	Less common on modern networks but still appears with VoIP and international calls.	More common because many users and businesses deliberately enable privacy.
Emergency Calls	Emergency calls rarely show this because carriers prioritise number delivery.	Emergency calls cannot use this because carriers override privacy for 911 or 112.
Business Impact	Businesses rarely see this unless using certain VoIP trunk configurations.	Businesses see this when customers or competitors deliberately hide their numbers.
Cost Implication	No extra cost; it is a by-product of the network's call routing path.	No extra cost on most carriers, though some may charge for per-call blocking.
Speed of Setup	No setup required because the condition arises from the network, not the user.	Setup takes seconds via *67 prefix or a permanent toggle in phone settings.
Accuracy	Label is accurate because the network truly has no number to display.	Label is accurate because the caller actively requested the number be hidden.
Durability	Persists for the duration of the call and disappears once the call ends.	Persists for the duration of the call and disappears once the call ends.
Scalability	Scales with network infrastructure; more VoIP routes may increase its frequency.	Scales with user adoption; more subscribers enabling privacy increases its frequency.
Maintenance	Requires no user maintenance because the network condition is automatic.	Requires no maintenance once the privacy setting is enabled on the line.
Safety Risk	Lower risk because the caller is not actively concealing their identity.	Higher risk because concealment often accompanies harassment or fraud attempts.
Compatibility	Works across all carriers and handsets because it is a standard network fallback.	Works across all carriers and handsets because privacy codes are universally supported.
Typical Users	Seen from automated systems, international callers, and legacy PBX equipment.	Seen from private individuals, small businesses, and privacy-conscious professionals.
Best-Fit Scenario	Accept it as a routing artefact and screen the call if you do not recognise the context.	Block it via settings if you receive frequent unsolicited private calls.

What Is Unknown Caller?

Unknown Caller is a caller ID label that appears when a phone network cannot retrieve or transmit any identifying information about the incoming call. It typically means the number is blocked, unavailable, or originates from a source that the carrier cannot classify.

Definition of Unknown Caller

Unknown Caller is a telephony status indicator displayed on a recipient's device when the incoming call's number and name are absent due to network limitations, international routing, or carrier-level privacy restrictions that prevent caller ID delivery.

Key Characteristics of Unknown Caller

Characteristic	What It Means in Practice
No number shown	The display shows "Unknown" instead of any digits, leaving you with zero callback ability.
Network-level origin	The absence originates from the carrier or switch, not from the caller's phone settings.
Common in VoIP	Voice over IP services frequently fail to pass caller ID data, producing this label.
International calls	Cross-border routing often strips number data, making foreign calls appear as Unknown.
Not user-initiated	The caller usually did not press a block button; the network simply failed to deliver data.
Inconsistent across carriers	One network may show Unknown while another shows the full number for the same call.
No name lookup	Your phone cannot query a contact database because no number exists to match against.
Hard to block	Most phones cannot block "Unknown" without also blocking legitimate hidden numbers.
Legitimate sources	Hospitals, banks, and government offices sometimes appear as Unknown due to their systems.
Distinct from No Caller Id	No Caller Id is a deliberate per-call block, while Unknown is an unplanned network failure.

Common Examples of Unknown Caller

Hospital switchboards – large medical systems route outbound calls through central servers that strip caller ID data.
International call centers – overseas support teams often appear as Unknown because their local carriers do not transmit numbers abroad.
VoIP business lines – small companies using cheap internet phone services frequently fail to deliver proper caller ID.
Government agency lines – certain federal offices mask their numbers for security, showing Unknown on your screen.
Automated appointment reminders – clinic software platforms sometimes send calls without embedding the clinic's number.
Satellite phone calls – calls from satellite networks like Iridium often lack standard caller ID data.
Old PBX systems – legacy office phone exchanges from the 1990s do not support modern caller ID transmission.
Ship-to-shore calls – maritime phone connections frequently fail to pass identifying information to landline networks.
Prison phone systems – correctional facility calling services intentionally hide numbers, appearing as Unknown.
Emergency service callbacks – some 911 dispatch centers call back with masked numbers for operational reasons.

Advantages and Limitations of Unknown Caller

Advantages	Limitations
Protects legitimate callers like doctors whose personal numbers would otherwise be exposed to patients.	Provides no way to identify or return the call, leaving you completely in the dark about who tried to reach you.
Prevents spam callers from being blocked easily, which ironically helps telemarketers who exploit the label.	Makes it impossible to screen calls based on known numbers, forcing you to answer every unknown ring.
Shields government agents and law enforcement officers whose identities must remain confidential during operations.	Cannot be distinguished from a genuine scam call, so you may ignore an important legitimate call thinking it is fraud.
Allows crisis hotlines to call back without revealing their physical location or exact phone line.	Blocks your ability to add the number to a block list, so repeat callers remain permanently unblockable.
Reduces the risk of phone number harvesting by third parties who collect caller ID data for marketing lists.	Fails to provide any verification that the caller is who they claim to be, increasing phishing vulnerability.
Preserves privacy for domestic violence survivors calling from shelters with hidden lines.	Creates anxiety because you cannot prepare for the conversation topic before answering the call.
Enables journalists to contact sources without exposing their personal phone numbers to the public.	Makes voicemail less useful because you cannot determine whether the message deserves a callback.
Prevents telemarketers from using your phone's auto-reject feature to bypass their calls entirely.	Offers no consistency, as the same caller might appear as Unknown one day and with a full number the next.
Protects confidential informants and witnesses whose safety depends on untraceable communication.	Cannot be reported to your carrier for investigation because there is no number or call record to trace.
Reduces the chance of your phone displaying a spoofed number that could be used against you in social engineering.	Forces you to choose between answering every unknown call or missing potentially critical communications entirely.

What Is No Caller Id?

No Caller Id is a caller ID feature that deliberately hides the caller's phone number and name from the recipient. It exists for privacy, allowing callers to control who sees their information. The recipient sees "No Caller Id" or a blank caller ID field instead of the actual number.

Definition of No Caller Id

No Caller Id is a telephony status where the originating caller has activated per-call or permanent line blocking, suppressing the transmission of their directory number and subscriber name to the terminating device. The receiving handset displays "No Caller Id" or a generic placeholder, preventing automatic identification and return-call functionality.

Key Characteristics of No Caller Id

Characteristic	What It Means in Practice
Number suppression	The full 10-digit phone number is stripped from the caller ID packet before delivery.
Name suppression	The subscriber's registered name is withheld, so no directory match appears on screen.
Per-call control	Users can dial a prefix like *67 before a number to hide identity for that single call only.
Permanent blocking	Carriers offer a line-level setting that hides the number on every outgoing call automatically.
Carrier-dependent display	Different networks show "No Caller Id", "Private Number" or "Blocked" depending on the provider.
Bypassable by recipients	Recipients can use *69 or call-return services to reveal the hidden number after the call ends.
Emergency call exception	Most carriers force number transmission on 911 or emergency lines regardless of block settings.
Voicemail interaction	Blocked calls often route to voicemail without a number, making return calls impossible.
Spam association	Legitimate businesses rarely use it, so recipients frequently assume the call is a telemarketer or scam.
International variability	Some countries ban anonymous calls entirely, while others require a mandatory unblock prefix.

Common Examples of No Caller Id

Domestic abuse survivors – They hide their number to prevent an abuser from tracing their new phone line.
Doctors calling patients – Physicians block their direct line to avoid after-hours calls from patients.
Journalists protecting sources – Reporters use No Caller Id so confidential contacts cannot be identified via call logs.
Debt collectors – Collection agencies mask their numbers to reduce the chance of recipients screening the call.
Police detectives – Officers hide their numbers during undercover operations or witness follow-ups.
Online marketplace sellers – Private sellers block their number when contacting buyers to avoid post-sale harassment.
Whistleblowers – They call newsrooms anonymously to report wrongdoing without exposing their identity.
HR departments – Recruiters block numbers when calling candidates who are currently employed elsewhere.
Prank callers – Individuals hide their identity to make anonymous nuisance calls without being traced.
Political campaign volunteers – Canvassers block personal numbers when making survey calls to voters.

Advantages and Limitations of No Caller Id

Advantages	Limitations
Protects personal privacy by keeping your number invisible to strangers and acquaintances.	Recipients frequently ignore or reject the call because they assume it is spam or fraud.
Prevents unwanted return calls, giving the caller full control over who can reach them back.	Call-return services like *69 can still reveal your number, so privacy is not absolute.
Useful for sensitive professions where revealing a direct line creates safety or boundary risks.	Voicemail systems often treat blocked calls as suspicious and may not record the message properly.
Free to use on most carriers with a simple prefix like *67 before dialing.	Many legitimate businesses refuse to answer anonymous calls, reducing your connection rate.
Helps maintain anonymity when contacting authorities or reporting crimes without fear of reprisal.	Emergency services may still display your number, so it offers zero protection in crisis situations.
Prevents your number from being logged by automated systems or telemarketing databases.	Some carriers charge a monthly fee for permanent line-level blocking, adding ongoing cost.
Gives you a clean way to make one-off calls without changing your main number or using a second SIM.	Recipients can activate "reject anonymous calls" settings, which block you before the phone even rings.
Reduces the risk of your number being harvested from caller ID logs by data brokers.	It damages trust in business contexts because clients and customers expect a visible callback number.
Useful for coordinating surprise events or deliveries where the recipient should not see the caller.	International calls with No Caller Id often fail to connect because some networks reject hidden numbers.
Protects your number when calling classified ads or marketplace listings from unknown sellers.	It creates a one-way communication barrier, so if the recipient misses the call, they cannot call you back.

Similarities Between Unknown Caller and No Caller Id

Shared Aspect	How Unknown Caller and No Caller Id Are Alike
Core Purpose	Unknown Caller and No Caller Id both hide the caller's phone number from the recipient's display.
Call Category	Unknown Caller and No Caller Id both belong to the category of blocked or private calls.
Display Output	Unknown Caller and No Caller Id both replace the actual number with a generic text label.
Caller Action	Unknown Caller and No Caller Id both require the caller to activate blocking before dialing.
Recipient View	Unknown Caller and No Caller Id both prevent recipients from seeing the originating phone number.
Network Standard	Unknown Caller and No Caller Id both rely on caller ID blocking standards in telecom networks.
Privacy Goal	Unknown Caller and No Caller Id both serve the caller's desire for personal privacy.
Caller ID	Unknown Caller and No Caller Id both suppress the caller ID information that normally transmits.
Mobile Support	Unknown Caller and No Caller Id both function on virtually all modern smartphones.
Landline Support	Unknown Caller and No Caller Id both work on traditional landline telephone services.
Carrier Service	Unknown Caller and No Caller Id both depend on the phone carrier to honor the block request.
Per-Call Toggle	Unknown Caller and No Caller Id both allow activation for a single outgoing call.
Default Setting	Unknown Caller and No Caller Id both require manual activation because neither is a default state.
Call Log Entry	Unknown Caller and No Caller Id both appear as unidentified entries in the recipient's call log.
Recipient Response	Unknown Caller and No Caller Id both often cause recipients to decline the incoming call.
User Base	Unknown Caller and No Caller Id both attract users seeking anonymity in communications.
Business Use	Unknown Caller and No Caller Id both are used by businesses protecting agent phone numbers.
Cost Factor	Unknown Caller and No Caller Id both are typically free features included with phone service.
Setup Effort	Unknown Caller and No Caller Id both require minimal configuration effort from the caller.
Technical Input	Unknown Caller and No Caller Id both require the caller to input a specific block code or setting.
Verification Limit	Unknown Caller and No Caller Id both prevent recipients from verifying the caller's true identity.
Spam Association	Unknown Caller and No Caller Id both are frequently associated with spam and telemarketing calls.
Trust Barrier	Unknown Caller and No Caller Id both create a trust barrier between the caller and the recipient.
Measurement Metric	Unknown Caller and No Caller Id both are measured by the answer rate they generate.
Carrier Removal	Unknown Caller and No Caller Id both can be overridden by carriers for emergency services.
Emergency Exception	Unknown Caller and No Caller Id both fail to block caller ID when dialing 911 emergency lines.
Maintenance Need	Unknown Caller and No Caller Id both require no ongoing maintenance once the block setting is enabled.
Global Availability	Unknown Caller and No Caller Id both are available across international phone networks worldwide.
Long-Term Outcome	Unknown Caller and No Caller Id both consistently result in lower call pickup rates over time.
Recipient Control	Unknown Caller and No Caller Id both give recipients the option to block such calls entirely.

Unknown Caller or No Caller Id: Which Should You Choose?

The deciding variable is who you want to hide from. Choose Unknown Caller for blocking telemarketers and spam while keeping your number visible to saved contacts. Choose No Caller Id for complete anonymity from every recipient, including people you know.

When to Use Unknown Caller

Choose Unknown Caller when you want to silence spam without hiding from friends. Use it for free call blocking on iPhones with no carrier fee, or when you still need your number shown to contacts in your address book.

When to Use No Caller Id

Choose No Caller Id when you need total privacy from every single recipient. Use it for calling businesses that sell your data, for anonymous tips or confidential reporting, or when your carrier charges for call blocking and you want a free alternative.

Common Misconceptions About Unknown Caller and No Caller Id

Common Myth	The Reality
Unknown Caller and No Caller Id are two different technical features on your phone.	Both labels describe the same caller ID restriction; your carrier simply displays one name or the other.
No Caller Id means the caller is hiding their number on purpose.	No Caller Id hides the number, but the caller may have set it accidentally or through a default carrier setting.
Unknown Caller always means the call is coming from a blocked or spam number.	Unknown Caller only indicates the number is unavailable; it does not verify spam, fraud, or any malicious intent.
Answering an Unknown Caller call will immediately expose your phone number to them.	Answering an Unknown Caller call does not reveal your number; your privacy settings remain active during the call.
You can call back an Unknown Caller number if you miss the call.	You cannot call back an Unknown Caller because the originating number is withheld and not stored in your call log.
No Caller Id calls are always free for the person receiving them.	No Caller Id calls are free to receive, but your carrier may charge you for specific call-forwarding or return-call services.
Unknown Caller is a brand-specific feature only found on iPhones.	Unknown Caller appears across Android, iPhone, and landline devices because it is a carrier-level label, not a phone brand feature.
No Caller Id means the call is coming from outside your country.	No Caller Id only hides the number; international calls can still display their country code and full number.
Blocking an Unknown Caller will stop all future unknown calls permanently.	Blocking an Unknown Caller only blocks that specific unavailable number; new restricted numbers can still reach you.
Unknown Caller and No Caller Id show different information on your phone screen.	Both Unknown Caller and No Caller Id show the same blank caller ID field; the text difference is purely carrier preference.
Your phone battery drains faster when you receive many No Caller Id calls.	No Caller Id calls use the same network resources as regular calls, so they do not cause any extra battery drain.
Unknown Caller calls cannot be traced by police or phone companies.	Phone companies can trace Unknown Caller calls through network records, but consumers cannot do this themselves.
No Caller Id is a setting you can only activate on a smartphone.	No Caller Id is a carrier feature you can activate on basic feature phones and landlines using a simple code.
Unknown Caller means the caller has run out of credit or battery.	Unknown Caller only means the number is hidden; the caller could have full battery and ample credit on their plan.
Receiving a No Caller Id call automatically saves the number to your contacts.	No Caller Id calls never save the hidden number to your contacts because the caller ID field remains completely empty.
Unknown Caller calls are always from telemarketers or robocallers.	Unknown Caller calls can come from doctors, schools, or friends using privacy mode, not just telemarketers or robocallers.
You need to pay a monthly fee to see who is calling from No Caller Id.	No legitimate carrier service reveals a hidden No Caller Id number for a fee; such offers are typically scams.
No Caller Id and Unknown Caller have different ringtone sounds.	No Caller Id and Unknown Caller use your standard ringtone; they do not trigger any distinct or special alert sound.
Turning off your phone stops Unknown Caller calls from reaching your carrier.	Turning off your phone stops all calls, but Unknown Caller attempts still route to voicemail if that service is active.
Unknown Caller is a sign that your phone has been hacked or compromised.	Unknown Caller is a standard caller ID label and is not an indicator of a hacked device or compromised security.
No Caller Id numbers are always blocked from leaving voicemail messages.	No Caller Id callers can leave voicemail messages, but the message will display as coming from an unknown or unavailable caller.
You can use a special code to unmask an Unknown Caller number instantly.	No consumer code can unmask an Unknown Caller number; only law enforcement with a subpoena can access that information.
Unknown Caller calls are more dangerous to answer than No Caller Id calls.	Unknown Caller and No Caller Id carry identical risk levels because both hide the same amount of caller information.
No Caller Id means the caller is using a VoIP service like Skype or WhatsApp.	No Caller Id hides numbers from any source, including traditional landlines, mobile phones, and VoIP services equally.
Your carrier charges you extra for every Unknown Caller call you receive.	Carriers do not charge extra for receiving Unknown Caller calls; they bill them as standard incoming calls on your plan.
Unknown Caller calls always fail to show up in your detailed call history.	Unknown Caller calls do appear in your call history, but they are listed as unknown or unavailable without a number.
No Caller Id is a permanent label that cannot be changed by the caller.	No Caller Id is temporary per call; the caller can disable the restriction and show their number on the next call.
Unknown Caller calls cannot be forwarded to another number on your phone.	Unknown Caller calls can be forwarded like any other call, but the forwarded line will also display the unavailable label.
No Caller Id calls always come from private individuals, never from businesses.	No Caller Id calls frequently come from businesses like clinics or delivery services that deliberately mask their main line.
Unknown Caller and No Caller Id affect your call quality or connection speed.	Unknown Caller and No Caller Id are purely labels; they have zero effect on call audio quality or connection speed.

Conclusion

Difference Between Unknown Caller and No Caller Id is simple: Unknown Caller means your carrier blocked the number, while No Caller Id means the caller hid it. Choose Unknown Caller to block your number from carriers. Choose No Caller Id to hide your number from recipients.

FAQs on Difference Between Unknown Caller and No Caller Id

What is the difference between Unknown Caller and No Caller Id?: Unknown Caller means your phone carrier could not identify the source number, while No Caller Id is a deliberate privacy feature the caller enables to hide their number from your display.
Is No Caller Id the same as a blocked number?: No, No Caller Id is a voluntary setting the caller activates to hide their number, whereas a blocked number is one you have manually added to your device's block list to prevent calls.
Which is safer to answer, Unknown Caller or No Caller Id?: Neither is inherently safe, but No Caller Id often indicates a legitimate caller using privacy features, while Unknown Caller frequently signals technical failures or potential spam networks, so screen both carefully.
Does answering a No Caller Id call cost me extra money?: No, answering a No Caller Id call uses your standard plan minutes and does not incur additional charges, although international or roaming rates may apply depending on your carrier.
Can I switch my phone to automatically reject Unknown Caller calls?: Yes, you can switch on Silence Unknown Callers in iPhone settings or enable Caller ID and Spam Protection on Android to automatically send these calls to voicemail.
Why does my phone show Unknown Caller instead of No Caller Id?: Your phone shows Unknown Caller when the network fails to transmit caller information due to technical issues, whereas No Caller Id appears when the caller intentionally withholds their number.
Are Unknown Caller calls more likely to be spam or scams?: Yes, Unknown Caller calls have a higher likelihood of being spam or scams because robocallers often use unregistered or invalid numbers that carriers cannot identify.
Can I use No Caller Id to hide my number when calling someone?: Yes, you can use No Caller Id by dialing *67 before the number on most US carriers or by changing your phone's outgoing call settings to hide your caller ID permanently.
Is Unknown Caller the same on iPhone and Android devices?: No, the labels differ because iPhone displays Unknown Caller for unidentifiable numbers, while Android typically shows Unknown or a blank field depending on the manufacturer's software version.
What should I do if I accidentally answer an Unknown Caller call?: You should immediately hang up, avoid sharing personal information, and then block the number in your recent calls list to prevent future contact from that source.