sorry for not looking at it earlier. I was in Thailand getting some sun.
Regarding yoru question, there are several reasons why softphoen would disconnect from TServer and the most obvious one would be bad network. I know that it is impossible in most cases, but check it.
Also, can you please dump the log from right before and after it? You should have commands like RequestRegisterAddress, etc in TServer log as well. I have noticed from log that you are using G3. Well, depending on how you register your DN type, your client would be kicked off by TServer.