If you forwarding a call out a ISDN port that is
busy it should send you a no channel available in the disconnect.
If you want to re-route on any failure, however, of play a busytone, you
can configure the isdn interface to send a release cause-code of 11.
So from global mode, use the command "isdn voice-call-failure 11"
The router A would receive this when isdn cause code is release for all reasons except
Normal call clearing, User busy, No user responding