I have this problem too. For me incoming calls to the LinPhone disconnect after 37 seconds. I do have a fix, but the fix is as bas is the problem.
My problems are on the IPhone. First problem was that it looses registration. But I learned that the LinPhone stays registered on IPhone if you set transport to TCP - fine - has to do with iOS support for TCP and not UDP.
However, incoming calls to the LinPhone disconnect after 37 seconds. This does not happen with any other SIP Dialer.
I have discovered that if I rewrite contact IP and port on the server side, then the disconnect problem goes away. However, once again the app looses registration shortly after it goes into background..
Anybody have any ideas?