That solution didn't fully resolve my issue. It turned out to be the battery. A second call got me to another person who sent me a new battery. The phone hasn't done it since.
AG
I was wondering about that. I was also wondering if it could be the SIM card. It just rebooted again a few moments ago, and it said on the lock screen "No SIM card. Emergency calls only." Then when it booted up, I could make calls.
It's an old SIM, from a 3-5 year old phone.
Edit: This may sound stupid, but how do you shut this phone off? I held down the power button for 8 secs, and it did a full reboot. (Not a quick reboot like is happening to me occasionally.)
Edit: Hmm, tried it again, this time holding it down for a few secs brought up a menu, and I was able to select shut down phone.
I cleaned the SIM card and what I could of the battery terminals with an eraser. The end of the eraser had black junk on it afterwards.