Here is my story: I had an iPhone 4 with iOS 5, which I used with iMessage. I sold my iPhone 4 to upgrade to a 4S, and before I sold it, I erased "all contents and settings" as described here: http://support.apple.com/kb/ht2110 I did not sync, restore from backup, or otherwise use the iPhone 4 after I erased it. At this point it should have in no way been associated with my iMessage account, and after wiping the phone until when I received my 4S, I had been using my SIM in an Android phone.
My wife started complaining that I was not responding to her texts. They showed as being sent via iMessage and all showed as 'received'. I blamed it on the Android phone, naturally, but the problem continued after I got the 4S, until I activated iMessage on that device. After that I did finally start receiving her texts, but also texts sent to and from the person who bought my phone. iMessage is also kind enough to sync messages he sent in response, so it looks as though I had sent them.
So, now I am quite sure this guy is receiving my iMessage texts, and seeing the ones I have sent via iMessage -- if I turn iMessage off on my new 4S, messages sent from my wife to me still go through iMessage, and are still marked as 'received'! If I keep iMessage off, I don't get my messages. If I leave it on, this guy can read all of my texts sent via iMessage.
Here is what I suspect is the problem: after I erased the phone, it still had my SIM in it and 'activated' with that (so that I could prove that the phone was still in working order), and the purchaser may not have put a new SIM into the phone. Even though I wiped the phone clean and did not set up iMessage, the phone still thinks it has my number, which iMessage must be identifying as me?
I called Apple support twice. The first guy informed me that I would have to get ahold of the person I sold my phone to, then walk him through restoring iOS. Great, so not only do I have to walk some stranger through restoring iOS, but then I have to explain to him that he can no longer use the phone until he gets a SIM, because of this restore that I made him do? So I called again -- the second person was a senior support rep, but had never heard of this issue, so hence this thread.
Is anyone else having this issue -- getting someone else's texts after selling their iPhone?