You can block your number when calling by dialing a carrier feature code before the destination number, by changing a handset or account setting that suppresses caller ID, or by using a VoIP or third-party service that rewrites the signaling. In the United States the near-universal code is *67, dialed once per call before the number. Permanent blocking goes through your carrier or phone settings, and none of these methods can hide you from emergency services.
What Caller ID Actually Transmits
Every call rides two separate channels that most people conflate. The first is the voice path, which carries audio once the call connects. The second is the signaling path, which sets up the call and, by default, passes your calling number to the receiving network before anyone answers. Blocking works on that second channel, not the first.
The signaling standard used across most modern networks is Signaling System 7, with caller identity handled through a service called Calling Line Identification Presentation, usually shortened to CLIP. The network inserts your number into the call setup message, and the receiving carrier decides whether to display it. Your *67 code sets a privacy flag in that same setup message. The flag tells the receiving network not to present the number to the called party.
Two more distinctions matter. Calling Line Identification Restriction, or CLIR, is the suppression instruction itself, and it comes in two flavors. A per-call restriction applies to one dialed number and resets afterward. A permanent restriction is a subscription setting stored on your line, so every outgoing call suppresses by default until you cancel it. The second distinction is between presentation and delivery. Presentation is what the recipient sees on the screen. Delivery is whether the receiving carrier still receives your number in the background. Blocking generally blocks presentation only. The network often still delivers your number to the carrier, which is why some parties can see it anyway.
Think of it this way: blocking flips a "display" switch at the receiving end, not a "send" switch at your end.
Why the *67 Code Works, and Why It Sometimes Doesn't
The *67 code is a star code, a short sequence your phone or switch interprets before the call leaves your device. When it works, the switch marks the call as restricted, and the receiving carrier's display logic honors the restriction. That handshake depends on both networks cooperating, which is where the fragile parts live.
Some carriers apply the restriction only to the number, not to the name. If your line has a name associated with it in a directory database, the recipient may still see that name while the number reads "Private" or "Anonymous." Others pass the call with the number intact but flag it as restricted, and certain enterprise phone systems ignore the flag and show the digits anyway.
There is also a structural limit you cannot dial around. Emergency calling uses a different path. In the US, the Federal Communications Commission has long required that 911 calls transmit a callback number regardless of any blocking setting, because dispatchers need to reach you if the call drops. Emergency services officials generally discourage trying to block on 911. If you call 911 from a blocked line, assume your number is visible.
A second structural limit: toll-free numbers and many business lines use automatic number identification, or ANI, to route and bill calls. ANI is a delivery-side feature that is separate from presentation, and it commonly reveals the number even when the display is suppressed. So a blocked call to a customer-service line is not private from the business, even if the human who answers sees no digits. This is the counterintuitive part most thin explainers miss: blocking decides what a person sees, not what the network knows.
Edge Cases That Break the Simple Answer
Blocking gets unpredictable in five recurring scenarios.
Line-pool and shared-number setups. If you call from a multi-line office phone system, the restriction may apply only to the primary line number. Calls placed from secondary lines or forwarded extensions can leak the underlying number.
Call forwarding and voicemail callbacks. When a call forwards, the forwarding party's number may replace yours in the signaling, or may ride alongside it. Callbacks from a voicemail system can also expose the number the system logged, which blocking never touched.
Voicemail and call recording. A recipient who does not answer may still get your number stored in the voicemail platform's metadata, even if the display showed "Anonymous." The suppression is cosmetic to the interface.
International destinations. Star codes are not global. Dialing *67 on a call to another country may do nothing, may be rejected, or may be interpreted as a different feature code entirely. Coverage depends on the originating carrier's roaming agreements.
Spoofed and relayed calls. Services that rewrite caller ID operate outside the normal CLIR path. When a third party or app substitutes a number, your carrier's restriction flag may be bypassed completely, because the call setup originates elsewhere.
Each case has the same root cause: the restriction flag travels with the call setup, and anything that rebuilds or re-originates that setup can drop or override the flag.
What Blocking Cannot Do: Persistent Identifiers
Suppressing caller ID hides one field. It does not hide the network-level trail the call leaves. Carriers log call detail records, which include the originating number, timestamps, and routing data, and those records exist whether or not the display was restricted. Law enforcement can obtain them through legal process. The FCC has repeatedly emphasized that blocking features are about consumer privacy on the display, not anonymity from the network or from lawful access.
Beyond carrier logs, the receiving carrier may retain the delivered number, and business systems may store it in customer records. The practical takeaway is limited and honest: *67 keeps your number off the typical recipient's screen. It does not make the call untraceable. Claims that a blocking code delivers true anonymity overstate what the signaling can do.
The Alternatives Compared
| Method | How it works | Hides number from recipient | Survives to business ANI | Setup effort |
|---|---|---|---|---|
| Per-call code (*67) | Sets a restriction flag for one call | Usually yes | No | One dial per call |
| Permanent line block | Carrier subscription suppresses by default | Usually yes | No | One-time carrier request |
| Phone OS setting | Handset marks outgoing calls restricted | Depends on carrier support | No | One-time toggle |
| VoIP account setting | Provider rewrites or withholds the header | Provider-dependent | Often no | Account configuration |
| Third-party app or relay | Routes the call through another number | Yes, shows a substitute | Shows substitute | App install and account |
The pattern is consistent: every method except a true relay simply flips a presentation flag. A relay substitutes a different number rather than hiding yours, which is a different product.
Enabling and Disabling It in Practice
Per-call blocking has a short time course and a specific unblock code, which trips people up.
| Step | Action | What happens |
|---|---|---|
| 1 | Dial *67, then the full number | Restriction flag set for this call only |
| 2 | Call connects | Recipient typically sees "Private," "Anonymous," or "Restricted" |
| 3 | Call ends | Flag clears automatically |
| 4 | Dial *82, then the number | Restriction overridden for that one call |
| 5 | Permanent block request | Carrier applies restriction to all calls until removed |
The unblock code is the easy-to-miss detail. If you have a permanent restriction active, *82 releases it for a single call, which matters when calling a party who screens anonymous calls. Emergency calls ignore both codes, as noted above.
Practical Actions and Thresholds
Use these steps in order, and verify before relying on any of them.
Confirm the star code for your carrier and country before an important call. In the US, *67 is standard, but confirm with your provider if you are on a smaller or regional carrier. Test with a second phone you control. Call it with the block active and check what the screen and the voicemail log actually show, because display behavior varies by handset. Decide whether you need a per-call block or a permanent one. Choose permanent only if you want every outbound call suppressed by default, and remember *82 to release single calls. Never assume a blocked number stays hidden from emergency services or business systems. Treat *67 as display privacy, not anonymity. Do not trust a third-party relay without reading its privacy terms, because a relay sees your real number on its own side.
One unit-system note for international readers: US star codes and the 911 requirement described here are specific to the United States. Equivalent features exist elsewhere under different codes and different legal requirements.
Limits of This Guidance
The mechanisms above are stable and well-documented, but carrier behavior still varies in practice. Display text, whether the name leaks, and whether business systems show the delivered number depend on the receiving equipment and the carrier's implementation of CLIR, and those details are not uniform. This article describes general technical behavior and does not guarantee that any specific call will appear private. For a legally binding answer about what a given carrier transmits, contact that carrier directly.
This article is general educational information, not a legal opinion or a guarantee of anonymity. Call-blocking features and the rules around them vary by jurisdiction and carrier, and lawful access to call records is governed by local law. For advice about a specific situation, consult a qualified local professional or your carrier. In an emergency, dial 911 or your local emergency number, and assume your number is visible to dispatchers.