It's possible that the OBi's SLIC module is defective, but more likely is that you made some configuration change to the ring profile for that service provider.
The definitive test would be to restore the OBi to its factory defaults by picking up the attached phone, confirming you hear a dial tone, then keying in ***8 then 1 to confirm resetting it to factory defaults. Wait plenty of time for the unit to reset and for the LEDs to stabilize (Left LED steady green, middle LED flickering to show internet connectivity).
At that point, you can reconfigure you can do drgeoff's test (plug a phone into each of the two phone jacks, press the # key on one phone's keypad, and the other should ring, and vice-versa.)
If it fails at that point, you have a defective OBi.