Following on from my post a couple of weeks ago. After speaking with Apple Support ( who agreed there was a problem but could not identify the cause ) they suggested I take the phone to my nearest agent of theirs which happened to be a EE shop.
EE had no specialist on site but sent the phone then to their repair workshop. I requested that whatever fix they applied they then tested the connection to blue tooth devices and most importantly to connect to hearing aids using the Accessibility option in settings.
They came back and said all was working fine, but they had "no hearing aids on site " with which to test that function.
Fat lot of help that was.
HOWEVER: The did 'flush' the software and upgrade it to IOS 17.1.1. Having got the phone home I set it up as new - i.e. did not restore anything from another phone nor from a backup.
Miraculously the phone connected to my hearing aids first go!!!!! So I then restored my other stuff from a backup and even more miraculously this did not upset the connection.
I use CAR PLAY in my car and that still works OK.
I have only made and received a few phone calls since the prone was returned and so far I have not experienced the call dropping nor do I hear any interference but it is early days yet.
The upshot is that it DOES SEEM to be a software issue and 17.1.1 fixes it.
Hope this helps the may sufferers out there.