asterisk users - May 2005 - How do you prevent a 3-way conference if an extension is busy ?

If I put an incoming PSTN call on hold, and then dial another extension,
which busy, I want to be able to use the hook-switch to cancel the
enquiry call, so that I can go back to the original incoming call.

Currently if I do this the incoming call gets connected to busy tone and
I lose control of the call. How can I prevent this behaviour ?