Caller ID is the telephone number presented to the called party so they can see who is calling before they answer. In the North American Numbering Plan (PSTN and interconnected VoIP), that is typically a 10-digit national number. The field is a presentation identity, not proof of identity.
On analog and SS7 networks the field is Calling Party Number (CPN), delivered with a presentation indicator (allowed or restricted). On SIP, the same idea appears in the From header and, when the originating provider asserts a network-validated number, in P-Asserted-Identity (RFC 3325). The number the handset shows is whatever the terminating network chooses to display after evaluating those headers, any privacy flags, and its own local policy.
Presentation is not identity
Anyone who controls an originating switch or SIP stack can populate a calling number. That is why Caller ID spoofing is easy in the absence of authentication, and why STIR/SHAKEN exists: the originating service provider cryptographically signs the number it claims. Attestation (A, B, or C) is a statement about the signer’s relationship to that number. It is not a name, and it is not ANI.
The number on the display can also be rewritten in transit. Some carriers overlay a billing or callback number. Some CPaaS and SIP trunk products let the customer set an outbound caller ID from a provisioned inventory of DIDs. The terminating side may still replace or hide it (for example when presentation is restricted, or when a spam label is applied).
Caller ID, CNAM, ANI, and STIR/SHAKEN
These four terms are often collapsed into “the name that shows up.” They are four different signals, stored in different places, with different trust properties.
| Signal | What it is | Who sets it | What the callee usually sees | Trust |
|---|---|---|---|---|
| Caller ID (CPN) | Calling Party Number presented to the called party | Originating switch, or SIP From / P-Asserted-Identity | The number on the handset (NANP: typically 10 digits) | Spoofable unless authenticated. See Caller ID spoofing and STIR/SHAKEN. |
| CNAM | 15-character caller name from a database dip (LIDB or a CNAM bureau) | The CNAM record associated with the number; the terminator decides whether to query | A short name next to the number, only if a dip succeeds and a record exists | Not authenticated. Often missing. Smartphone contacts override it. |
| ANI | Automatic Number Identification used for billing and some terminating features | Originating network billing identity (historically Feature Group D) | Usually not shown to the subscriber | More stable than CID. *67 can hide Caller ID and still leave ANI intact. |
| STIR/SHAKEN | SIP identity signature and attestation (A / B / C) over the calling number | Originating service provider signs; terminator verifies | Not a display name by itself. May feed analytics, branded calling, or spam labels | Designed to reduce unsigned spoofing on IP-originated calls. |
Blocking, contacts, and smartphones
Calling Number Delivery Blocking (Vertical Service Code *67 in the NANP, 1167 in some areas) sets the presentation indicator to restricted. The called party’s phone typically shows “Blocked,” “Private,” or “Unknown.” That flag affects Caller ID presentation. It does not erase ANI on the billing path, and it does not apply to E911 callback identity.
On smartphones, the address book wins. If the incoming number matches a contact, the device shows the contact name regardless of the network CNAM string. If there is no contact, the phone may show the raw number, a CNAM result if the carrier supplied one, or a carrier spam label. Nothing in the North American call path guarantees that a business name will appear.
NANP display vs international CLI
Caller ID as a consumer feature is defined most tightly in the NANP (country code +1). International Calling Line Identification (CLI) follows national rules and the ITU-T numbering plan in E.164. A number that is valid as an E.164 address can still fail to display if the terminator expects a 10-digit NANP number, if the originator sent a national format without a country code, or if a privacy rule in the destination country strips CLI.
- Outbound Caller ID should be a number the originator is authorized to use (a provisioned DID, local number, or toll-free number).
- Name display is a separate lookup: see CNAM.
- Attestation is a separate signature: see STIR/SHAKEN.
- Spam labeling and robocall analytics sit on top of all of the above.