If power-cycling the OBi doesn't stop the repeated rebooting, try using a paper-clip through the hole in the bottom that has a circle around it. Hold the reset button down for at least 30 seconds before releasing it. If the OBi stops repeatedly rebooting, it should be back at factory defaults and ready to be reconfigured.
If the rebooting continues, does it stop if you unplug the Ethernet cable? If so, try this:
1. Dial *** 0
2. Enter option 27 and press #
3. Press 1 to set a new value
4. Enter a value of 1 and press #
5. Press 1 to confirm/save
6. Hang up
7. Wait for the OBi to reboot