Call flows that operators actually trace

Rooms

1. Two different ACK rules

   The ACK for a non-2xx is hop by hop inside the INVITE transaction, and the ACK for a 2xx is a new end-to-end transaction.

2. Record-Route and the route set

   A proxy that wants to stay on the dialog prepends a Record-Route URI, and that URI has to contain lr.

3. PRACK and reliable provisionals

   A provisional response with Require: 100rel is retransmitted until PRACK, and RAck names that exact response.

4. UPDATE before the INVITE completes

   UPDATE can change the session while the initial INVITE is still outstanding, and it does not use the ACK rules.

5. Session-Expires and 422

   Session timers name who refreshes the dialog, and 422 tells the caller the interval was below Min-SE.

6. CANCEL, BYE, and re-INVITE

   CANCEL abandons an unanswered INVITE, BYE ends a confirmed dialog, and re-INVITE changes a dialog that already exists.

7. REFER and Replaces

   REFER asks another UA to send a request, and Replaces names the dialog that request should take over.

Ecosystem

SIP does not replace workforce management, quality monitoring, CRM, emergency policy, or a messaging registry. It is the session seam those systems hang from. Compliance notes on this page are educational and are not legal advice.

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