During SIP calls, various request and response messages are exchanged (see Request methods and Response methods). For example, a SIP call is started by the caller sending an INVITE request to which 180 Ringing and 200 OK responses are expected.

These request and response messages contain various 'headers' detailing different information values, see Headers. Some of these headers contain contact information in the form of SIP URIs (Uniform Resource Identifier). For example; the caller, the original destination, the current destination, and so on.