I would second the Grandstream recommendation. I have used the 502,503,4004,4104 and am in the process of installing some 4008 boxes. I have mostly installed them for T.38 faxing support in a private network environment, but my current project is to provide some backup dialtone on a multisite mpls networked phone system.
Albeit my install was connecting to Mitel systems, the fundementals aren't much different. My current project has them connecting back to an Adtran 916e that is sharing a PRI between remotes using the Grandstreams and the host a PRI. The customer is loving the roi since the Adtran was free from their mpls provider and this will be running via vpns off their customer wireless access isp connection.