If they insist you can do it, start insisting that they can run like Usain Bolt. When they say no they can't, tell them that they ran when they were younger, they can still run really fast and beat Bolt, just like you can really hear well now.
All joking aside, tell them seriously that they are not treating you fairly when they say those things. Not all disabilities are static, yours has changed and they are greatly disrespecting you by not accepting how you describe your abilities. They are saying they don't believe you know what you can do, and that goes for your skills as well. If they accept that you can type (put in whatever skill set you can do here), then they need to accept that you cannot answer the telephone. It's as simple as that.