Fax modem not transmitting voice signal

Hi group.
My machine is Vista Home Premium SP2. It came with a CXT PCI Soft Data Fax Modem installed, and Realtek High Definition Audio.
My question is this; Shouldn't this modem be able to transmit voice signals over an analog phone line? When I connect my phone line to the RJ11 connector of the modem I am able to use windows phone dialer to dial out, make a connection, and I can hear the party on the other end, but they can't hear me. I have tested the microphone by recording my voice on windows sound recorder, so I know it works. What am I missing?

Dave T.