Assuming cell tower gremlins, I didn't think too much about it, but as a precaution returned home and restored and re-synced my phone.
Next day I made another call or two with no problems, then patted myself on the back for dealing with the problem on my own. Then the same symptoms returned. Not only were outgoing calls failing, but incoming calls failed in the same way.
A trip to the Apple Store was in order, so I made an appointment and sauntered on in. The tech was helpful. Tried the usual tricks like restarting the phone, resetting network settings etc, but nothing solved it. We switched SIMs with another phone, but no joy there. He suggested I call AT&T and have them 're-provision' the phone. Did that but no dice.
It looked like it was finally time to replace the phone, so he pulled my serial number and had a kind of funny look on his face. He asked for reinforcements, and soon a manager joined my tech as they looked increasingly bewildered while viewing my account details.
They asked me if I had had problems with the phone before? I said no, it has been great. They asked me if I ever had a new replacement shipped? No again. Had I ever failed to return a replaced phone? Nope.
I started to feel slightly guilty about all this. It was like something out of Kafka. Perhaps I had done these things and blotted them out of my mind, but the reality was it seemed like they were talking about someone else. They said their records showed I had many problems with the phone, and that Apple has dispatched a new one, but I never sent the old one back, and even worse, that would make replacing my phone problematic at best.
I was flabbergasted, but the manager guy said he was going to the secret Apple back room and would return soon. While waiting, my mind spun up a hundred scenarios, all ugly. Could I possible have forgotten all these problems? Was I a victim of identity theft? What would I do if they said they wouldn't replace the phone? Etc, etc.
After about 40 minutes he told me had made several calls and determined that someone else with my name in another state had created the long trail of problems that had somehow wound up under my name and iPhone serial number. One can only wonder how that could have happened.
The story ended happily, and the Apple store guys really went the extra mile to get to the bottom of this, rather than just dust me off and say 'tough luck'. I wonder how many retail chains would do the same?
Lessons? Computer systems and people aren't perfect. Somehow my name and iPhone were attached to someone else, and without a lot of determined effort I would have been sunk, probably forced to buy a new phone and left to try to repair my incorrect service record. I don't know if the help I received from my Apple store was typical, but a great effort was made to trust me and get to the bottom of the problem. If it is typical, it helps explain the high rankings Apple gets for customer service. My nightmare ended happily. May all your nightmares be little ones that are easy to fix.