When a call is offered, the destination exchange can present the calling user’s number to the callee, along with extra address information such as a calling-party sub-address when that information is available. ITU-T Recommendation Q.731.3 specifies this as a supplementary service for the called user. The value on the display is the Calling Line Identification (CLI) carried in signalling across the Public Switched Telephone Network (PSTN), ISDN, or Voice over Internet Protocol (VoIP) using the Session Initiation Protocol (SIP).
How the Number Reaches the Callee
The destination local exchange handles the call as usual, then the user-network interface checks whether the called user has subscribed to presentation. The network is expected to deliver the identity at set-up even when the terminal cannot show it.
- Called-User Subscription: The service is offered to the callee, not switched on by the caller. If the complete number is not available at the destination exchange, digits are not presented and a presentation indicator can mark the identity as unavailable.
- Calling-Party Number: The originating local exchange or the caller’s access signalling supplies the number. When a generic number marked as an additional calling party number is also present, that additional number is presented first, then the calling-party number.
- Signalling Path: On ISDN the identity travels in the signalling channel. On analog lines it is often sent as a modulated burst between ring cadences. On VoIP the same presentation is carried in SIP signalling (commonly From or P-Asserted-Identity, subject to Privacy).
- Name Versus Number: This service presents digits, not a brand string. Caller Name Delivery (CNAM) is a separate lookup. Outbound teams typically present a Direct Inward Dialing (DID) they own rather than a neighbor’s number.
Restriction and Emergency Override
Calling Line Identification Restriction (CLIR) takes precedence over presentation. A callee who has subscribed still does not receive the digits when the caller has restricted the identity, unless a national override applies.
- Restriction Wins: When restriction is applicable, the originating network notifies the destination that the number must not be shown. The callee may receive a presentation-restricted indicator instead of digits.
- Override Category: Some networks define an override for designated called users, commonly police or other emergency services. That national option lets the callee receive the number even when restriction is set. Whether override exists, and how it works across networks, is a national matter and may require bilateral agreements.
- Forwarded Calls: A forwarded-to user who has subscribed to presentation still receives the original calling number if the original caller has not invoked restriction.
- Presentation Is Not Authentication: Showing a number does not prove it is authentic. Secure Telephone Identity Revisited / Signature-based Handling of Asserted information using toKENs (STIR/SHAKEN) attests that the originating carrier stands behind the identity being shown.
CLIP, CLIR, and CLI Compared
The three terms travel together in ITU-T number-identification signalling. One is the identity, one is the right to show it to the callee, and one is the right to withhold it.
| Aspect | CLIP | CLIR | CLI |
| What it is | Supplementary service that presents the calling number to the callee | Supplementary service that withholds that number from the callee | The calling identity itself, typically the number plus screening and presentation indicators |
| Offered to | The called user | The calling user | The originating line (the payload, not a subscription) |
| Default outcome | Digits shown at set-up if the identity is available and not restricted | Digits hidden; a presentation-restricted indicator may be sent instead | The value the presentation service would display |
| Precedence | Yields to restriction unless the callee has an override category (national option, often police or emergency services) | Takes precedence over presentation | Not a service; it is the identity those services present or hide |
