This is pretty cool if you're using a cell phone on a budget or just like to talk all the time for long durations.
Xcelis, a new service, currently only available for Cingular / AT&T customers offers unlimited calling for your cellphone at 9.99 a month.
Works with any plan with unlimited mobile to mobile as its relays are appaerantly classified as mobile phones on the cellular network.
Check it out, thought it was cool.
http://xcelis.com/