If anyone's still monitoring this forum, I thought I'd share a success with getting the phone book to dial contacts using a SIP service. There's a TAPI driver available at
When configured with your own SIP account, and with the driver selected in the "Dial using" box, when you click "Dial", a call is made to your registered IP phone, then to the called number, as with an operator-connected call.
The driver version for Organizer is 0.2.6, released in June 2009. I have it working in Vista, 32-bit, with Organizer 6.0.