Jeroen over Grynx has done a very good DIY on building your device to connect your normal phone to PC. There is a similar device on the market called Chat-Cord but it is quite expensive – so why not make your own:
Not too long ago I ran across a device called Chat-Cord (www.chat-cord.com).
This device does actually the same thing but it is placed between you phone and pc, not modifying your phone. But… This device is pretty expensive and I couldn’t get it here in the Netherlands. Furthermore it seemed to me that this device actually isn’t very complicated. So, after some internet research I somewhat found out how it worked and identified two difficulties to be solved. In this article a description is given how to make your own chat-cord. It costs only like 7 euros. You have to solder some parts but it is very basic and simple.
To be able to use a normal phone to connect to the pc we have to make it look like for the phone as if it were connected to a normal telephone line and this telephone line has to look like it is making a call.