Personal IP Telephony Gateway?

three55ml asks: "I'm interested in putting together a personal SMALL scale IP telephony gateway to the PSTN (telephone network). I have a server with a voice modem in the area code I want to make calls. Are there any solutions for Windows or Linux that let me integrate a simple voice modem and conferencing software to place calls via the Internet? I know about products like LineJack, but I would think using a simple voice modem this could be accomplished on this small of a level."