I went to the AT&T Store.
They said there were 3 possibilities:
1) Reset network settings.
2) Change SIM card.
3) Antennas on phone have degraded and it's time for a new phone.
We tried resetting network settings.
I'll see if this makes a difference.
If not, I'll try option 2.
However, resetting the network settings lead to another problem. I couldn't enable Wi Fi.
This caused a day of trouble shooting, and multiple calls to tech support.
The solution ended up being ANOTHER reset of network settings.