You would normally set up the OBi to send 911 out on Callcentric and other calls on GV.
In addition, I recommend having it send toll-free calls on Callcentric as well. If sent via SIPBroker, these calls are free. That results in the CC service getting some regular use, so you know it's working, should it ever be needed for 911. Upon request (and at no charge), CC will set up your service so the outbound caller ID matches your GV number. You can always force a particular call to use one service or the other, by prefixing the number with **1 or **2.