Outlook Phone Dialer Audio Problem

[Abi]

For no apparent reason, the phone dialer on my Outlook no longer
allows me to hear calls. My headphones I use work for all other
sounds, and I even tried a different pair, just to be sure. The calls
go through (I've dialed our cell phone to test it.) I just can't hear
the dial tone, or anything.

I've run every diagnostic in Office XP I can find, and the modem as
well as the headphones have passed.

Any ideas appreciated!