Using a USB modem for voice calls

Hi,

I purchased a Zoom 56K V.92/V.90 USB External Modem (Model 3095) for the sole purpose of using it to make voice calls from my PC via Outlook. The modem works from Outlook, it dials and on my headset I can hear it ringing at the other end and what answered I can hear the person on the other end however the person on the other end cannot hear me. I have been through all the setting I can find relating to the mic on my headset (which works fine with Skype, MSN Messenger, etc...) and everything seem to be in place.

I assume that somewhere there is a setting to tell the modem to use the mic on the headset, but I'm blowed if I can find it. Any suggestions gratefully welcomed.

Thanks in advanced.

Andrew.