We had a situation that I've written about before that might be worth checking. Make sure that the Inbound and the Outbound calls are coming and going from the opposite ends of the T1 channel. If in coming calls are coming in on 1,2,3 make sure that outgoing calls are going out on 23,22,21 (or whatever).
If they are coming and going from the same part of the T1, you have collisions that could be knocking out the channels and causing your busies.
Adam in DC