Definition
A DID number, short for direct inward dialing, is a phone number that outside callers can dial to reach a specific line, extension or voice agent directly. Many DIDs can share one connection to a carrier.
How a DID works
A business used to buy one line per phone. A DID breaks that link. The carrier rents you many numbers, all delivered over a single connection, and tells your system which one was dialed. Your system then decides where the call goes.
With a SIP trunk the dialed number arrives in the SIP INVITE, in the request address and the To header. A phone system or agent platform keeps a table from number to destination: 415 555 0100 to the sales agent, 415 555 0101 to support. The numbers follow the E.164 format, a plus sign, the country code and the national number, so a US number is written +14155550100.
You get a DID by renting it from a carrier or platform, or by porting a number you already own. Porting moves the number between carriers and can take days, so plan it ahead of a launch.
Why it matters for voice agents
A DID is how a caller picks an agent. One number per agent, per campaign or per location lets the platform load the right instructions without asking the caller anything. A separate number for each marketing campaign also tells you which one produced the call.
For outbound calls the number matters in the other direction. The number you present is usually a DID you own, and recipients decide whether to answer based on it. A number that recipients have flagged as spam gets answered less, so teams often keep several.
Coverage differs by source. On Cartesia, numbers issued by the platform are US only, while numbers on your own SIP trunk can come from any country your carrier serves.
Example
An illustrative setup for a dental group: three DIDs on one trunk, one per clinic. The platform reads the dialed number, loads that clinic’s hours and booking rules, and answers with the clinic’s name. All three share the trunk’s channel limit.