Modem in use by other application

I've got a Dell 8300 w/internal faxmodem, running Windows XP SP3. When I try to send a fax, using the Microsoft software, the app indicates the modem is in use, but I can't find any other such app running.

I tried to remove the modem in Device Manager, and allowed PnP to detect it. No issues there, but the faxing problem is still there.

I tried PhoneTray software (voice mail/callerid) and it works fine, w/o the line in use error. Just in case, I disabled this software in startup. So it seems like the modem is OK. If there really is a mystery app using the modem, why does PhoneTray not  have the problem?

Anyone have any ideas?