Chatcord

A Chatcord is a low cost and compact device that connects the duplex audio stream between the sound card of the computer and the telephone-set. It enables the use of a Plain Old Telephone System (POTS) to talk via the Internet away from the computer, after connection is established from the desktop using your specific softphone (such as MSN or Skype).
A number of cheap and compact devices exist and also many Do It Yourself web pages where instructions for assembling one are given, as well.
A quick web search on 'chatcord' will yield numerous results, both as DIY schematics and commercial.