You can swap the SIM, but At&t will require an iPhone data feature at a minimum of $15/month once you start using the iPhone. You will keep your same mobile # and voice plan when you switch phones.
Is it possible to use a sim card from my current phone on an iPhone?
I have a Sony Ericsson phone and my carrier is AT&T, I want to switch the phone to an iPhone. Can I take off the sim card and put it on the iPhone? Will it work? I'm thinking of purchasing a used iPhone 3rd genaration on CL.. but first I want to know if it will work, I just want the iphone to use to take picts, play some apps and take some calls sometimes. Also does the phone have free wi-fi like the iTouch?