My texts are going out but I get a "Not Delivered" error message

I have the new iphone 5. It started giving me error messages saying my texts were'nt delivered, but they were. I went to the genius bar and they reset my phone, restored it and I lost all my stuff. But it worked for about a half day, then the same red error message "Not delivered" appeared. The thing is, most of these error texts were delivered, but not all. So i went back to the Genius bar and they gave me a new phone. Then I went to ATT and they gave me a new sim card. So far so good. Texts were going through perfectly, then today, I got another text with the message, so called the recipient, and they had received the text. I have imessage turned off. Now what ? I have a new phone, a new sim card, I reset the phone, reformatted the phone. Whats the next step? I'm guessing its a iphone 5 bug and eventually they'll fix it but its annoying and disconcerting when you don't know if they received the message or not. I sent a happy birthday message to my husband 5 times because of the error message and he got all of them. Give me a break. Any ideas ?