When my AT&T iPhone 4S recently came off contract, I began to look for alternative plans. I originally picked Red Pocket, because it meant I didn't need to jailbreak/unlock my phone. Red Pocket are an MVNO reselling AT&T airtime, so their sims should work in any AT&T phone. I ordered a MicroSIM from their web site for $9.99. It arrived in a few days, I popped it in, went through the Activate page on their web site, and within minutes I was setup with a new phone number. I could have also transferred a number, but since I use Google Voice, I always just get a new number.
