As far as the regulations I think it depends on the state. For example our cluster is in Texas and I have to provide the floor with our CER. If your MPLS circuit fails you would probably fail over to the local gateway running CME and a pots line for...
Physical phones we have used are both 7970 and 7975. We could not get the 7937 conference phone to work and we have not tried any of our Gen2 7940 phones. Hope this helps.Good luck.