How to get Iphone 4s connected to straight talk?

I bought a used Iphone 4s that is supposed to be unlocked but when I put my straight talk sim card in it (from my current straight talk phone), it says "no sim". I've left it in for over an hour, powered the Iphone off/on but it still doesn't recognize my sim card. Does this mean that the phone is locked? How would I go about transferring my current straight talk service to the Iphone?

It has to be an AT&T iPhone that was unlocked, or one that was originally unlocked when new.

But it won't work if it's a Verizon or Sprint iPhone 4S: even though those have SIM cards, if you unlock them, all you're doing is unlocking for international use so that the phone will work with non-USA SIMs when it's outside the USA. But inside the USA it won't use the SIM at all and will still use CDMA instead.