Initiating a SIP voice call from Outlook Autodialler

Hi:
I'm in the process of designing the communications infrastructure for
my company and VoIP and SIP plays a big part. Likewise IM using
Windows Messenger (or MSN Messenger - though I believe the latter
doesn't support SIP).

My latest experiment has been to add the details of my SIP/VoIP
provider into Windows Messenger. In a wondrous demonstration of how
software should work (!!) I signed out and back in and Lo! I am able to
make voice calls to other SIP addreses or (via my provider's gateway)
to POTS phone numbers. All works wondrously well.

Since I have geographic phone number I can even receive incoming calls
straight into my Messenger window.

The icing on the cake will be to initiate a call via the SIP
Communications Service of Messenger from my Outlook contacts.

I can't find any way of telling the autodialler to use the SIP
communications service.

All help gratefully received!

Thanks in advance