I have been experiencing this many times. Incoming calls get disconnected at ~13 minutes. I don't know about outgoing call though. What could be wrong? The firmware version of my OBi100 is 1.3.0 (build 2651). Thanks!
A good first test is to eliminate your router as a possible source of the problem. Connect the OBi directly to your modem and power both units off. Bring up the modem first, followed by the OBi. If the problem doesn't go away, you will know your router is not at fault.
There is also a firmware update available (Build: 2669) that you probably should install before doing anything else.