The two most likely reasons:
1. You did not select Google Chat as one of the forwarding destinations on your GV account settings page at google.com/voice. (Your OBi acts as a Chat client.)
2. There is currently a problem with incoming GV calls to some OBi devices. There are several posts and threads about that on these forums. See eg
http://www.obitalk.com/forum/index.php?topic=13680.0. That thread has posts by SteveInWA and one contains a link to a thread on the Google forum which has more advice.