Remote Office with Survivability

I would like to setup 2 remote offices with about 8 Polycom phones at each remote office. All offices will be connected with an MPLS 1.5meg T-1 connection. I also would like to have survivability and local dial tone at the remote sites. I have done some research and found that I should be able to take a Adtran 908e or 6310, or 6355 to connect a local PRI and turn it to sip trunks that would provide SIP trunks to the Switchvox located at the main location under normal operation. I am being told that if the network goes down the Adtran box can redirect incoming calls to a “default” phone, it would also allow 911 calls to route local directly through the PRI with out going to the Switchvox PBX. I am being told that it will also allow calls to route directly through the Local PRI if the network goes down. My question is has any one done this yet??? I do not want to put separate Switchvox systems in due to the fact that you lose the seamlessness of one system. If anyone has done anything like this please let me know how it is working.