I have Costco KS9 hearing aids (Phonak made). I’m experiencing a similar issue. Search for a thread I started “KS9 bad connection on phone calls”.
No solution yet for my issue. I believe it may be related to iphones 8 and newer having Bluetooth 5.0. Phonak marvel & ks9 use Bluetooth 4.2. Everything I have found 5.0 is backwards compatible to 4.2.
I have not updated my thread but the static is back on my hearing aid phone calls. I just went back to my 2016 iPhone se to see if static issue is related to new iPhone se 2020.
If you find a solution please post.