Works like this, you have linmodem/winmodem with a voice driver, or a voice modem. You run this p2p app and it lists the exchanges that are a local call for you. If someone wants to call a number local to you, you would route the call for him.
This has the normal p2p problems (freeloaders, problems with the law, 'but officer I didn't make those calls.. I swear'), and plus I don't think 14 year old hackers parents would understand why people they don't know keep talking other people they don't know on their phone line.
One solution to the parent problem is to put two linmodems in the 14 year old hacker box and reroute all the home phone onto this network (maybe this could be done with one linmodem, don't know). BBS kids used to jack up the household phone bill, now they kill all long distance changes :).
It would be intresting to see how quickly you could put this together reusing as much code from other things as possiable.