Public Mobile has asked me to select a 'solution', to help others with this problem. But I cannot because none of the suggestions 'alone' solved the problem. However, most of the suggestions helped lead me to solve the problem. This is the answer: After going through all the steps suggested and not solving the problem, I came to the conclusion that the problem had to be with the iPhone settings. And the quick answer is "Focus". If you are not using it, or very seldom, just delete it. But it cannot be deleted! In my case, a setting that silenced the phone for most times of the day got turned on, somehow. It seems to have happened after an update. I could not accept a phone call during those times. By discovering this little detail, I solved the problem. So.... the problem was not with Public Mobile, but with the iPhone feature called Focus. Hopefully this helps someone.
