Using my PC as a POTS telephone?

TheJerbear79 writes: "I recently accepted a work from home job that will involve using my landline to talk to customers. When I log into the phone queue my landline will ring, I'll put in a three digit code, then calls are routed to the phone call I'm on. It essentially turns my landline into a softphone. Rather than using a regular handset, or obtaining a nice business phone with a headset and speakerphone and all that, I would like to use my PC's modem in conjunction with a normal PC headset and soundcard. I know the hardware is capable, but the modem didn't come with appropriate software. Have any slashdotters found anything cheap/free that would suit this application? Just for clarity I don't want to use a VOIP solution, I want to use my plain old landline. My reason is if I'm watching a movie or listening to an mp3 while I'm waiting for a call, I don't want this to EVER be apparent to the person I'm on the phone with and I want to route all the audio I use through a single headset. I've scoured google for anything close to this application and all I've managed to find is information on VOIP software, and software that turns my PC into an answering machine. Neither of which will work for what I'm trying to do, so I'm calling in the cavelry!"