2026-03-08
In traditional telephony, caller ID is set by the originating carrier based on the line's registered number. In VoIP, the caller sets their own caller ID in the SIP signaling. This fundamental difference is what makes caller ID modification possible with VoIP.
The primary header that carries caller ID information. Format: From: "Display Name" <sip:number@domain>. The number in the URI becomes the caller ID, and the display name appears on phones that support it.
Used between trusted SIP entities to convey verified identity. Some carriers use PAI instead of or in addition to the From header. Format: P-Asserted-Identity: <sip:number@domain>
An older header used by some systems (notably Cisco) for caller ID. Format: Remote-Party-ID: <sip:number@domain>. Less common in modern deployments but still encountered.
When a VoIP call reaches a terminating carrier (the one delivering the call to the recipient), the carrier extracts the caller ID from SIP headers and passes it to the recipient's phone through the SS7 or ISDN signaling.
CLI routes are configured to pass the caller ID through without modification. The terminating carrier displays whatever value was in the SIP headers. No CLI routes may strip or replace the caller ID before delivery.
STIR/SHAKEN is a framework implemented in the US and Canada to authenticate caller ID. Calls receive an attestation level (A, B, or C) indicating how confident the originating carrier is about the caller's identity. This doesn't prevent caller ID modification but adds a trust layer.
CLI routes use carriers that are configured to pass through caller ID without filtering. These carriers have agreements in place that allow caller ID presentation, which comes at a premium. No CLI routes use cheaper carriers that may strip or replace the caller ID.
The From header is the primary SIP header that sets caller ID. Some systems also use P-Asserted-Identity (PAI) or Remote-Party-ID (RPID) headers.
No. STIR/SHAKEN adds attestation levels to calls but does not block them. Most international calls receive C-level attestation. Recipients may see a verification indicator but calls still go through.
Each terminating carrier handles caller ID presentation differently. Some look at the From header, others at PAI. Some carriers perform CNAM lookups that can override the displayed name.