Session timer

Session timers (RFC 4028) keep a SIP dialog alive by requiring periodic refreshes. An INVITE that includes Supported: timer and a Session-Expires header proposes a refresh interval; the 200 OK either accepts (returning its own Session-Expires and naming the refresher with refresher=uac or refresher=uas) or declines. The named refresher sends a re-INVITE or UPDATE at half the agreed interval; if a refresh doesn’t get a 200 in time, the other side sends BYE; Reason: SIP;cause=408;text="Session timer expired".

The most recognisable failure pattern is “calls drop at exactly N minutes”, where N matches half the agreed Session-Expires. The investigation reads the 200 OK to find the agreed interval and the named refresher, then traces whether the refresh was sent and whether it reached the far side. Stateful middleboxes dropping the refresh INVITE are a common root cause; trunk profiles without session timers enabled are another.