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