Well, I might have a different experience here (also in France, using Orange): I’m now inside Paris, with solid 5G coverage everywhere, and my FP4 is now continuously on “5G”.
Since I’m here, the annoying choppy sound issues I had down south have gone away altogether! 
Sound is like it should be - crystal clear, on both sides, in any situation.
Now could it be (potentially, just a wild guess) that something inside the FP4 chokes on anything slower than a solid 5G connection? Emphasis is on “solid”: When I was on borderline situations where the phone switched between 5G-4G, I had choppy sound.
It would be consistent with my experience so far. What do you guys say?
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Edited to add, two weeks later:
No, the problem came back, so 5G and the quality thereof are not a factor. The problem seems totally random, just coming and going randomly, sometimes for a longer period.