Some possibilities I can think of that could cause this:
1) RTP fails to be established, and so the call drops.
2) Malfunctioning SIP ALG.
3) Electrical problem with your phone.
Let's start with 1). Navigate to Service Providers >> ITSP Profile X >> RTP. Make a note of LocalPortMin and LocalPortMax.
Using your router, forward the UDP port range LocalPortMin-LocalPortMax to your OBi. If your OBi does not have a static IP, you will need to assign one to it.
Let me know if that works, or not.