I rather wonder if some parents do that so there'll be a boomerang effect, counting on kids to come back around for meals and such, or even move back in.
With phones, you'd best know what you want to use the phone for and how much you want to use it. If you don't use your phone much for advanced computery things like games and video, a bare-bones phone with a pay-as-you-go plan might be fine. If you and the person you want to talk to are computery anyway, Skype is free and will save you both some minutes on your plan.