I am in the process of updating some 7100's from a sip trunk configuration that was done before the Metaswitch Interop guide came out
I am going through this process as I was never able to update beyond FW R10.6 without issues on the sip trunks, as I was using an unsupported configuration I figured I should configure via referenced document.
So updated to R10.8.1, and configured the sip trunks on the Meta and the Adtran as specified in the document. Everything seems to work generally, phone to phone calls work, phone to external calls work, external to phone calls work... but External calls that use FMFM or call coverage to ring external numbers will NOT work.
So call into PBX - Call coverage - outbound call to XYZ = No Audio Call rings can be picked up, but no audio either direction.