For Callpilot, if its only meant to go through to one of the mailboxes (say the call the helpdesk and get put through to an app) with the thru dial you could try puting in Extension DN 2 as the number used on the SDN (Service Directory number).
I'm probably not following your question properly though as you asking for thru dial when someone gives a number a call, almost like a self service reception which means the destination is multiple phones.
It's going to be a problem from what i know though as the original call is destined for the number of which its answered by callpilot, not the thru dial number.
Trent Steenholdt
Systems Engineer
The University of Notre Dame Australia
-Great scott!