Hi,
Busy out on ISDN is not easy as it may seem.
When the call comes, in the setup message is already indicated a channel, either exclusive or preferential. That is, is the switch that decides on which channel the calls lands.
However, certain switches support service messages to allow individual B channel busy out.
Check:
http://cisco.com/en/US/products/ps6441/products_feature_guide09186a0080686068.html