asterisk users - Feb 2016 - Handle a call if one phone of a ring, group is busy

I do it via a group count:

main call handling:
exten => sub123,n,Set(GROUP()=11122345)
...
the main routine calls subroutine:

exten =>
general,1,GotoIf($["${busyonbusy}"="YES"]?100:200)
exten => general,100,GotoIf($[ ${GROUP_COUNT()} > 1 ]?110:200)
exten => general,110,Hangup(17) ; fehlercode 17 = SIPcode 486=user busy here
exten => general,200,Return()
...


that works as well and you can specify how many calls are allowed.
regards,
andre