said by tiger72:if you're on a carrier (like T-Mobile) which offers smartphone data-only plans (without voice plan requirements), then you can grab a skype-in number, load up your line with cheap minutes, and just use your data plan and wifi for all of your calling needs.
People talk about this all the time, but I've found that the voice quality of Skype for Android when calling most cell and landline phones, even from a strong wifi signal, is so poor that it is completely unusable and I have to call people back using a cellular call. So as much as I'd love to relegate my carrier to being a dumb pipe, and making all my network usage generic 'bits' that they can't control as much, the services for it just aren't there yet.