If the phone has decent wifi capabilities, you can use that. It's faster, anyway. Phone calls will work. I've got both unlocked Android (Apollo) and Nokia C6. I mostly use my own wifi at home. Have Skype on both phones.
If you are after TMUS, you can go to a store and get a bring your own phone plan.
You can only get it where a human can explain it to you. It's cheaper, a lot of people jumped to it, found out they could no longer get a subsidized phone and raised h***.