Sound quality when calling gradually deteriorates
I'm having a sound quality issue with my iPhone XS, running iOS 15.5 (it's up to date).
What happens is when I'm talking with somebody on the phone (just voice, not facetime), the sound quality gradually deteriorates. Essentially, I can't hear the person I'm talking to well enough after some time (the call starts fine) -- the sound is breaking up, the quality goes down, I only hear some words, etc. The person in the other side hears me just fine. If I hang up and place the call again, the problem disappears for a few minutes, and then the quality of sound goes down again. It doesn't seem to depend on the carrier, mine or the person's I'm calling.
What can I do to get this fixed? Already tried restarting and updating the phone, turning the speaker on and off. None of that fixed the issue.
Thanks,
iPhone XS, iOS 15