I'll second what WanderingTraveler said- My wife's 521 doesn't have a data plan so we do everything by WiFi. It works well if you don't need to be constantly connected to the internet.
Just make sure you have T-Mobile disable data on their end, turn off data on each phone (under Settings/Cellular), and I set Data Sense to the minimum, 1Mb per month, just in case there's a glitch and something gets through.