semi-VOIP idea

I was hoping I could receive some help here on this project I thought up. Currently with the program AIM Triton, you can request a phone number and then whenever someone dials that number your aim screen rings.... thankfully aol took care of all the complicated things ie. linking the phone number to your screen name.... what I want to do is I hope much simpler. Using a voice capable modem .... I want to build a program that will route the audio thats playing through the speakers to the modem.... and take any incoming audio from the modem and route them as if they were coming in through the pc's microphone..... the result would enable the user to plug in a cordless phones base to the computers modem and when the program realizes that aim talk has started it will start the routing and allow the user to pick up the cordless phone and start talking to the person through the computer. ANY help would be GREATLY appreciated! if people are interested and want to work together on it we can start an open source, source forge page.
-aarona567