I wouldn't resign, I'd take advice from ACAS. If you do have a hearing problem, something can be done about that, and your employer is obliged to accommodate that. Could you ask for headphones to use on the phone so the distractions don't impair your hearing callers? - we do.
Is there more to this than meets the eye? If you will tell us, what was the improvement warning about - are they trying to go down the incapacity route (i.e. you're no longer up to the job)? Very hard to prove and you would need to be given a period during which you have to demonstrate improvement, if you have done well in earlier years.
Is this a new manager, new broom trying to change things?
If they sack you, you have every right to go to tribunal.