I would like to see some snips of your config. By your description, I would look at the following:
1) Do your sh isdn status, debug isdn q931....make sure your D channel is functioning properly.
2)I'd also like to see your dial-peers. They may be a source of trouble as well.
Have you performed a debug voip ccapi inout on calls that are busy? What is the gateway sending out?
-Mike