Hi,
It is strange. The call type is identified in the q931 setup message by the bearer capability.
Values:
88: ITU-T coding standard; unrestricted digital information
90: Circuit mode, 64 kbps
21: Layer 1, V.110/X.30
8F: Synchronous, no in-band negotiation, 56 kbps
Try to make q931 debug (debug isdn q931) when the call fails to see what happens exactly.
Hope it helps, rate if does
Krisztian