Call Manager Express + isdn pri

Could anyone assist. Im running Call manager express 3 on a remote router. I have an Isdn pri line using 30 channels for voice. Callers sometimes find isdn lines engaged yet there wont be anyone on the phone. where could my problem be?

Othertime I had a similar problem, but instead of isdn pri, I had 2 voice cards with 4 ports connected. I was getting 7 out of 8 possible simultaneous calls. i had missed out defining ephone-dn for one of the ports as well as putting an overlay for the 2nd button on my ephones (7940).

Thanks in advance