Definition
Caller ID is the calling number, and sometimes a name, that the network sends with a call so the recipient's phone can display it. The sending carrier supplies it, which is why it can be wrong or faked.
How caller ID works
The calling number travels in the call’s signaling. In SIP it appears in headers such as From and P-Asserted-Identity, and the carrier on the far side hands it to the recipient’s phone. The name is different. In the US it is usually not carried with the call. The recipient’s carrier looks the number up in a database to produce the name, called CNAM, so the name shown can differ between carriers or be missing.
Nothing in the basic protocol proves the number belongs to the caller. A phone system can put any number in the header, and many have legitimate reasons to do so, for example presenting a main office number on calls placed from several lines. That is why carriers added signing, covered under STIR/SHAKEN.
The law cares about intent. The FCC says the Truth in Caller ID Act prohibits transmitting misleading or inaccurate caller ID information with the intent to defraud, cause harm or wrongfully obtain anything of value. The Telemarketing Sales Rule separately makes it a violation for a telemarketer to fail to transmit its phone number and, where the carrier makes it available, its name, though it allows substituting the seller’s name and a customer service number that is answered during business hours.
Why it matters for voice agents
For outbound agents, the number shown decides whether people answer. Calls from numbers that recipients or analytics apps have flagged as spam often go unanswered. Use numbers that you own or are authorized to use, give each one a clear business purpose and answer return calls on them.
For inbound agents, caller ID is a hint, not proof. Do not treat it as authentication. An agent that discloses account details because the number matches a record can be fooled by anyone who spoofs the number. Verify identity with something else.
Which number appears is controlled by the carrier and the number configuration, not by the agent’s instructions.
Example
An illustrative failure: an agency places outbound calls from numbers that no one answers when called back. Recipients see an unfamiliar number, ignore it, and later search it online and find complaints.