Why can't my iPhone receive the confirmation code?

I haven't received the confirmation code on my cell phone. I entered my cell phone number, but I haven't received the confirmation code yet. I use AT&T and so I entered that carrier. But its the same thing as Cingular, so when I didn't receive it after asking for it twice, I changed the carrier to Cingular, but still haven't received it. Am I doing something wrong? :(