1.
See if your phone has a reset of some kind. It may be a tiny hole where you insert a toothpick.
2.
See if you can extract the battery and clean the contacts. Scour them with a pencil eraser. Steel wool requires cautious use because it risks a short circuit.
3.
My cellphone started giving me problems and I believe the reason was because I remained 3G in a 4G world:
* sometimes unable to find a tower when I wanted to make a call,
* frequent loss of connection,
* garbled, faded words during calls.
Problems stopped when I acquired a 4G smartphone. I also had to pay more for a new plan.