On a free or paid CC account, you can make free toll-free calls by sending e.g. **275*18005551212. This routes the call via SIPBroker and does not require two-stage dialing. From an OBi not specially configured, you would have to dial e.g. **2**275*18005551212.
However, you can configure the OBi digit map to do this automatically, i.e. you would dial 18005551212 and the OBi would automatically select SP2 and send **275*18005551212. The result would be that all toll-free calls would go via Callcentric and SIPBroker, which is IMO a good thing, because you'll know the CC service is working, should you ever need it for 911.
The 911 service is configured for your physical address and does not directly depend on your phone number. If you call 911, in addition to your address, a special temporary number is displayed to the dispatcher, which they can use to call you back, in case you get disconnected or hang up inappropriately.
Having nothing to do with 911, you can set up your home number (whether or not ported to GV) as an outbound caller ID for Callcentric, so any calls you make with their service will show that number to the callee.