So I'm fairly technology-challenged and I'm sure someone here has to know this. I was thinking about getting one of those unlocked cell phones you can buy on Amazon. It says you can just stick in the SIM card you already have and start using it, but I remember the last time I got a new phone it wouldn't work, and it turned out the reason why was I had the wrong kind of card. (This might have had something to do with the fact that the "new" phone was really cheap and crappy and presumably a much older model, I don't know.) So how do I know if the one I have will actually work?