Need a quick answer about SIMs!

I tried doing several searches but I'm just not figuring out the right keywords.

I'm about to buy a phone on craigslist. Decent deal that I want to jump on. It's a 3G 16GB that is currently running 2.2.1

I have an AT&T contract through work. Will I be able to plug in my work sim and have it function properly? Or will there be other steps that I would need to do first?

Quick answer would be much appreciated.

Thanks

To the best of my knowledge if your a AT&T subscriber all you do is put your sim in and your good to go. I Have a phone with 2.2.1 with modem firmware 02.30.03 on it and my sister in law came over she's a subscriber to AT&T and I put her sim card in and I called here number and my iphone rang so I believe it works I also called out on it. Hope this Helps