I am trying to configure a cisco router to be the interface between a PBX and the WAN link. I have a PBX connected to a Adtran. The Adtran then goes to the DSX1 of a CSU/DSU and the NI lead on the CSU/DSU is connected to an ethernet which serves as the WAN link to carry the voice from the PBX to a remote site. The CSu/DSU has a serial interface that goes to a serial card on an old cisco router. I am trying to replace the CSU/DSU and Adtran with a C2821. Is this possible? If so what module do I use on the router and how do I go about with it?