[Asterisk-Users] Limit concurent calls per MSN on BRI (bristuff/zaphfc)?

[Pisac]

How can I limit incoming concurent calls per MSN on BRI ISDN?

I'm using bristuff/zaphfc with imediate=no in zapata.conf, and I have 4
MSNumbers. I want that incoming caller (to my MSNumber 1234) get busy if
that number is already in use through another B channel, so that other B
channel is available for incoming calls to other MSNumbers. I don't want
that 2 incoming calls to MSN 1234 invade both B channels and congest my
ISDN.

Any solution?