I suggest looking at the STATUS buffer to see what happens with the call. You should probably clear the status buffer first (go into STATUS and hit "0"), then either dial the call or receive the call.
I'm assuming this is an ISU 512. If so, how many channels are supposed to be connecting? If it negotiates 6 channels (for 384k) and not all the channels connect, then the call will fail. This is the same for both dialing out and receiving calls.
With the ISU answering, verify the LDNs are all correct, because this is what is used during the BONDING negotiation to tell the dialing side what it needs to dial for the additional channels. (Local LDNs do not effect outbound dialing.)
If all the calls are answered but there is a Bonding timer that expires, you may need to adjust the timers on the ISU.
The STATUS buffer is going to be best indication of what is happening during the call setup.
Hope this helps,