IMS

SIP Parameters : Call-ID

As it sound, Call-ID is an identification to represent a specific call session(a Dialog more specifically). The Call-ID header field is an identifier used to keep track of a particular SIP session.

This can be very useful for troubleshooting especially a trace log contains multiple transactions among multiple UAs. We need to be able to keep tack of each sessions separately. In this case, you can track down/separate out each sessions using Call-ID. When a call originates from UA_A, UA_A generates its own call id and when a call origate from the UA_B, UA_B generate its own call ID.

When you see 'From', 'To' address for various messages and if the Call-ID are the same, you can think all the message belong to the same call(Refer to 3GPP 24.229 4.10.1 General).