Not interly sure i follow
If you want to divert the main number to a mobile then the call forward third party if fine
I have never had a problem with it and we use isdn2 / 30 and sip trunks
You may be talking about forwarding at the exchange...then you dont want call forward third party
It is just for forwarding another number instead of your own
It would go feature code/ multicall key/ destination, dont forget the 9 if to an external number
Can do it with multicall and hunt groups not ring groups.