I have been facing a quite strange problem with my E51. For the moment I have configured a SIP provider for connections over UMTS/HDSPA. Everything seems initially to work. It will open a packet connection, register to the SIP provider and make a call just fine.
Just after having hang up the first call, no more calls will be completed. On every attempt to start a new call, the phone simply says "Connection Error". The connection manager will show that the packet connection is still "alive", but the connection seems stalled (every 28 sec there should be a stun refresh, no more data flows over it). When I try to open an website using the same APN, the webbrowser will refuse it claiming "System Error".
All I can do is to manually terminate the packet connection and trigger the next call, after the call the packet connection will be dead again. This really sounds like a bug, can anyone confirm it? Is there a workaround?
Btw, over WLAN the same provider will work just fine. And I have already tried every setting availiable in the sip configuration app.