[VoiceOps] GRE

I have a project in which a user is interesting in interconnecting and deploying ATAs for VoIP services but wants to use private IP addressing on the devices. My initial though is to NAT these devices and allow my SBC to handle NAT traversal for them. This allows the to use their dedicated connection to my network or if that link goes down fail over to moving to connecting across the internet. They are interested in potentially doing a GRE tunnel and keeping the private addressing in place as it traverses that tunnel. I am concerned about encapsulating the VoIP traffic as well as with the fail over options. Does anybody have any experiences with VoIP over GRE tunnels and what type of results and challenges you encountered? Thanks. --BH