The only thing that I have found that WORKS is power your iPhone off and then on each morning. This has worked well for me. I’m a light user so someone on the phone all day may have a different result.
I have Marvel M90s with an iPhone 11 Pro.
The phone power cycle worked for me this morning on a test call to a robot voice (a weather report). Before the power cycle I was getting static on that source. What also works for a test on that same phone number is what @mipresser reported: turning off the left HA. My Bluetooth connection is with the right HA, which is the programming default.
These two “solutions” seem to me to be unrelated. One would seem to affect the phone; the other, the connection between the right and left HAs. Puzzling.