[asterisk-users] Asterisk dial plan conditional on not busy

[Michael]

Here is the current dial plan section:

[custom-michael]
exten => _900,1,Playback(custom/extn-xfer)
exten => _900,2,SayDigits(${EXTEN})
exten => _900,3,MixMonitor...........
exten => _900,4,Dial(SIP/${EXTEN}|${DEFRT})
exten => _900,5,Playback(custom/extn-xfer2)
exten => _900,6,Goto(custom-michael,901,4)

exten => _901,1,Playback(custom/extn-xfer)
exten => _901,2,SayDigits(${EXTEN})
exten => _901,3,MixMonitor..............
exten => _901,4,Dial(SIP/provider/${CELL1}|${CELLRT})
exten => _901,5,Goto(vmail|900|1) <-- go to voice mail context

DEFRT = 30 seconds, CELLRT = 25 seconds.

Currently if the first phone is busy, or is not answered within 30 seconds, it 
will jump to the next phone (a cellular phone) and try that for 25 seconds 
before going to voice mail.

What I want it to do is only try the cellular phone if the extension '900' is 
not answered. If busy I want it to go straight to voicemail.