Baolaniyan,
It is doubtful that it is the autoswitch because if the autoswitch is configured and active, typically you will not notice that the call has been switched over. It will not drop the call because of autoswitch. Additionally, AS is default to disable and you have to configure both ends of call (which both must be Tenors) to activate it. If the call is ending in the middle, you should check your IP network to see that it is stable first.