I bought at&t without contract and i can call how is that possible?

I bought iphone 4s AT&T from amazon for $380 it arrived with SIM card preinstalled and I can call. How is that possible I didn't purchase any plan!

Added (1). It was factory sealed as the seller claimed, so I don't think that someone forget his SIM in it!

Maybe it was used and someone didn't cut it off yet

It is an unlocked phone. You can search it on google, it basically means you can use it with multiple carriers.

Most AT&T locked iPhones that are did not come out recently are under $1.