Hi, I have an iPhone 3G 8GB, that I recently picked up from a friend. I have no information about the history of the phone, other than that he said it was working a month ago, and then it was left sitting for a few weeks and it put itself into a DFU mode loop. I was able to resurrect the iPhone from the DFU loop, and I hacktivated it in order to be able to use it to test the functionality. It's currently running iOS 3.1.3, because I didn't want to update yet until I knew that everything I wanted to do was available on the newer firmware versions (4.x). So basically, the iPhone has been hacktivated and jailbroken. Now I myself have verizon, so I have no sim cards to test with the iPhone. My question is this:

Will the iPhone be functional on an existing AT&T plan, if I simply insert the existing plan's sim card into this iPhone? I'm a little confused because I've read that once hacktivated, an iPhone must be unlocked before it can be used with ANY carrier. I simply want to test the iPhone to make sure the call functionality works. How would I go about testing it? Do i need someone with a sim card that has the iPhone required 3G data plan? Do I need to restore it back, and activate it through AT&T? Do I need to unlock it? Swift assistance is requested, HELP!