Help - custom call forwarding to external voicemail

I have set up an H323 connection to an external Voice Messaging system (using the tried & trusted method of trial & error - mostly error!).

I can dial the pilot number & it will answer correctly. What i need to do now is set up some way of forwarding busy & no answer extensions to this voicemail instead of the internal one. I am still struggling with the config syntax, so any pointers or help would be appreciated.

I need to be able to pass the called party ID, calling party ID & some sort of forwarding reason code.

Any ideas??