For no apparent reason my Snom phone ends the current call after about 15 minutes!
To Understand the issue let's take a look at the following SIP communication between Snom Phone and PBX:
- The Snom Phone with number 123456 sends an Invite to 7890;
- The call comes in and is established between 123456 and 7890, so far so good.
In this case it is very important what is negotiated between the Snom phone and the other party (PBX server).
- Let us concentrate on the parameters that are important for the duration of the session ( call ).
- The Snom phone suggests a session duration of 3600 seconds; see: Session Expires: 3600
- In the 200 OK the PBX changes this value to Session Expires: 1800 (=30 minutes).
- Additionally it defines who renews the session: refresher=uas. This means that the (uas: user agent server) takes over the session refresh. In this case the PBX.
What does the RFC say: The refresher has to send an update after the half of the time. In our case it is exactly 15 minutes.
- If the client (in this case the Snom phone) does not receive any message from the server after half the time to keep the session running, the client terminates the session.
- What the Snom phone also does with the sending of the BYE.
- This procedure is described here: https://tools.ietf.org/html/rfc4028
Here you can see the BYE being sent by the phone after 15 minutes and 13 seconds:
In the SIP Trace we also see that right after the Snom phone sends the BYE, the PBX sends an update for the session, but this comes late(13:12:43.583).
- This proves that the problem is on the server side, because it does not update the session in time.