I have had this conversation at work at least ten times in the last month.
"Ellen, your phone is breaking up. I can't hear you / you sound like you're underwater / it's all garbled / what are you saying?"
I then yell into the phone, "IS THIS BETTER?" and the people claim it is, but they're likely frightened by my annoyed tone.
The Apple Genius Bar ran diagnostics and found nothing.
Gary, the hero, found the answer. Evidently, if your phone says it is using 5GE cellular, that is a lie. In 5GE, the “E” stands for “Ersatz”. Evidently this imaginary 5G (that doesn’t exist in the U.S. yet, you lying marketing bastards) is worse than 4G. Garbled, twisted, underwater calls.
If I see 5GE at the top of my phone now, I just send an email.