TAPI

Microsoft uses TAPI (Telephone API) for placing calls from Outlook (or dialer.exe)

To take advantage of dialing from Outlook with an SIP phone you need a SIP TAPI driver.

I've tested out a few, and SIPTAPI from IPCom does the job. http://www.ipcom.at/en/telephony/siptapi/

You install by copying siptapi.tsp to the %systemroot%\system32 directory.

Then launch the Phone and Modem Options from Control Panel (assuming Win XP).