Welcome to this user forum.
Assuming you can still make outgoing calls, it sounds like a fault within the exchange equipment. You appear to be making no progress with this fault.
I have asked a moderator to provide assistance, they will post an invite on this thread.
They are the only BT employees on this forum, and are based in the UK.
Thanks for getting in touch. I'm sorry to hear that incoming calls aren't ringing the handset.
You seem to have carried out most steps that we'd need you to try so I'd like to take a look into it. I'll need your account details to test everything. Click on my user name and then copy and paste the link under "Moderation Team Email Address" into your browser. Complete that form and we'll get back in touch with you.