If the phone is "world ready" you should be able to swap sim's without unlocking the phone, but if you bought through Best Buy and they're telling you that you can't do it without unlocking, then I'd check directly with your carrier. A sim should only cost 5-6 bucks, not 20, but you still have to buy minutes. Probably cheaper to buy a cheap Lime phone at Value Master (under $30 out the door, including minutes).