When my wife and I upgraded phones, we gave our old smart phones to our kids so they can play with them. With Google Voice and GrooveIP, they now have their own phone numbers, can text, make calls over WiFi for free, and not have to deal with any ads. The only charge is for GooveIP which is $5. There is a free version, but it is limited. The phones are not signed up with any service.
EDIT: And GrooveIP integrates with the native dialer, which I like a lot.
Here are two comments I couldn't help but notice. If the comments are true, there might be better apps that do the same thing.
"Its not bad but when someone msg me i dont get a sound or notfication. I have to be inside the app and refresh to ne able to see the new msg. Horrible."
"This app is awful. I receive texts from so many people that I have no idea who they are. And the worst part is the people won't stop texting me asking if this is "sartakk".
It seems like I don't really get notifications."