VoIP glossary · Calling Line Identification
What is Caller ID (CLI)?
Caller ID, called CLI (Calling Line Identification) in much of the world, is the phone number presented to the person you are calling.
Caller ID (CLI) explained
On a SIP trunk the caller ID is carried in the From or P-Asserted-Identity header and should be in E.164 format. Whether the number actually reaches the handset intact depends on the route: CLI routes are expected to deliver it, while cheaper non-CLI routes may replace or drop it along the way.
Many countries now regulate which numbers you may present. You should only send numbers you own or are authorised to use, and several regulators require carriers to block calls with invalid or spoofed caller IDs. Delivered CLI also affects answer rates, since people are far more likely to pick up a familiar local number.
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