What they are basically doing is VoIP (internet packetized bit stream)to the customer site then dropping the voice calls out on the DSX interface. A 64K "B" channel for a voice call can be compressed (read: squeezed)to about 7.2K, which when you add the added overhead for the Packet header and control/routing packets works out to about 8k of bandwidth per call. It does depend on which compression Codec they use, there are several. This is one of the basic reasons data (and modem) calls won't work worth a dang over IP, you cannot compress a data call and expect it to work. If you use the G.711 codec (64k-uncompressed) then you can send a "data" call, but guess what?, it takes 80k of bandwidth (16k of overhead) to send the call via IP, not 64k, you do the math.
Mode1, this is why your "voice only" dynamic PRI works "ok", In the Magix you can choose "voice" or "data" or "both" in the PRI setup, I would bet the "working switch is "voice" and the 'iffy' switch is "both"??? It takes the switch additional time to negotiate the call-setup if it is set to "both" because the Adtran has to reach a compromise before 'it' can route the call also.
By the way, you could have different provisioning set up in the Adtran's too. As you can see this can get complex quickly.