I want to make telephone calls through my computer using my Bluetooth headset. I tried this with my laptop, I could hear the person I dialed (using windows phone dialer in XP), but they couldn't hear me. So I figured maybe it's because I don't have a voice modem. So I bought a speakerphone modem for my desktop computer, and tried again, but this time there's no sound at all with either Windows Phone Dialer or the BVRP phone tools that came with the modem. Has anyone done this or know how this might be done?

Windows phone dialer isn't designed to do more than dial;

Usually, speakerphone modem will have mic input on the modem itself - doesn't get the audio via the soundcard/PC soundsystem.
If the bluetooth headset goes to soundcard, you should be able to connect line out of the soundcard into the mic in of the modem - that would let the called party hear you - but to hear them via the headset could be problem...