Read the negotiated timer on the affected leg
Session-Expires carries the session interval and can identify the refresher role. Min-SE expresses a minimum interval. Inspect the successful negotiation on the particular dialog; a configured PBX value alone does not prove what the two peers accepted.
Create a small timeline with the final answer, agreed interval, refresher, first refresh attempt and final termination. For a B2BUA, repeat this on the other leg. The carrier-side agreement and the phone-side agreement need not be identical.
Look for a successful refresh
A session refresh uses re-INVITE or UPDATE. A successful 2xx response refreshes the timer; merely sending a request does not. RFC 4028 recommends attempting refresh around the midpoint of the interval, leaving time for failure handling. Other qualifying session changes can also refresh it.
If a call ends near a repeatable boundary, identify the last successful refresh and what happened to the next request. Did it leave the refresher, reach the peer, receive a response and return to the originator?
Separate three unrelated lifetimes
Registration controls a reachable contact binding. An RTP inactivity setting watches media reception. A session timer refreshes signaling session state. Changing a phone’s registration interval is therefore a poor experiment for a failed in-dialog refresh.
If audio stops before the call terminates, capture both timelines. A later BYE may be cleanup after an earlier network failure. Conversely, continuous audio does not establish that a refresh request was routed successfully.
Correct the demonstrated negotiation or path failure
A 422 response points to an interval negotiation problem. A rejected overlapping change may require the 491 investigation. A missing response requires delivery and routing evidence. Keep the exact response and dialog identifiers in the case record.
After a correction, test beyond at least one full negotiated refresh interval and include hold/resume. Record the successful refresh response and ordinary hangup. Avoid declaring success from a short call that never reached the former failure boundary.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.
Examples require adaptation to your topology. No live PBX or hardware testing is claimed.