There are many ways to do this, Google Voice being chief amongst them. Where things really get interesting if when you want calls to come in. The best option

There are many ways to do this, Google Voice being chief amongst them. Where things really get interesting if when you want calls to come in. The best option I've found is to use Google Voice and link it to a Skype In account. It may not be the best solution, but I can run my smartphone on wi-fi only and save a ton of battery this way.