Some suggestions were drastic including a full restore of the iPhone, and others were annoying by suggesting that you call at&t customer support. All that noise sounded like way too much of a headache to me, so I just did a simple power cycle (turn off the phone, then turn it back on), and that fixed it. Amazingly simple. I’ve made it a habit to start with that as my first step of troubleshooting my iPhone, computers, routers, modems, cable boxes, etc. Sometimes it fixes it, and sometimes it doesn’t. It’s nice when it does.