Or the user who actually uses a phone to make calls.
I don't know if it's the phone or the network - I haven't done much testing. It's quite irritating pressing the answer button repeatedly and the phone not answering the call, or when I call someone and I end up unable to hear them while they can hear me.
Of course these are isolated data points. I do think that Apple has made a mistake by limiting the iPhone to one carrier, especially one that can arouse strong hatred like AT&T.