[USR-TC] ISDN from a Cisco1604 won't connect second channel to HiPerDSP.

We are trying to connect a Cisco1604-ISDN Router to a TotalControl HiPerDSP. One channel comes up perfect, but the second channel is dropped immediately whenever it tries to connect. We can connect the 1604 to a Cisco3640 that has a BRI card in it, and it does 2 channels no problem. Also, we can also connect a 3Com Lan Router to the HiPerDSP and both channels work fine (even on different spans.) Cisco-to-Cisco is fine and 3Com-to-3Com is fine. But Cisco-to-3Com doesn't work. Go figure! Anyone have any suggestions? Thank you, -Wayne