The phone is only compatible with At&t and T-Mobile, At&t may automatically add a data plan if they notice it on their network. You can use the device on wifi without any data plan, don't even need a sim card.
I have been using mine on At&t with the 200MB $15 plan for 9 months now, I have never gone over 75M in a month, but MMV when it comes to data. I am on wifi 95% of the time, so my data usage is very low. It might not work for someone that is constantly on a 3G network.
You might also consider the pre-paid carriers, if there are any that are compatible, even At&t might work as they then can't add a data plan.