What kind of multiplexers do you have between the facilities. That may be an option, by replacing the interface cards on one side of the network. Multiplexers are being ripped out left and right these days and very easy to find affordable replacement cards on the used market.

When I briefly scanned the post i was assuming a back to back connection (nortel-mitel), but this cant be the case if there is network involved. So im thinking by replacing cards on the multiplexer on the mics end could do the trick.

~n