How do I learn to use the telephone?

I'm terrified of using the telephone. I never answer calls unless I'm sure it's my wife calling and I never call anyone unless it's the most desperate emergency. Making appointments for doctors or car repairs weigh heavily on me and I keep putting them off. I've tried services for the deaf, but that's just more embarrassing because they have to explain my problem before getting to the reason I called.

I'm tired of being humiliated on the phone. You see, people always ask me to speak up. It kills me when they say that because I don't want to have to raise my voice and shout and make them angry. I don't understand why they can't hear me.

You appear to have a communication phobia when dealing with people you have never met. This phobia can only be defeated through practice and self confidence. When making calls jot down what you want to say then respond in a normal conversational manner. Practice with your wife and friends and tell them you want to be more self confident in dealing with others. Your question is well written so now all you have to do is transfer that to speaking skills.

I don't fully understand your problem.
Do you talk quietly? Do you have a physical defect/deformity that hampers your ability to hear or speak?
Is your fear of phones purely psychological?
Can you see a therapist or a speech therapist?
They might be able to practice on the phone with you to improve your anxiety.

How about getting one of those Darth Vadar voice modulators?

I think that would solve your problem. Very few people would dare to complain to Darth Vadar.

Well you can get a phone with a mic volume control, that might help with that issue

Have you tried speaking with a therapist?

My understanding is that it isn't uncommon among people with social phobia to have difficulty with the phone. Often overcoming these phobias is a mater of gradual desensitization, do something that is just a little bit scary, practice it until it isn't scary anymore, and then make it a little harder. Baby steps, just a little at a time.

A good therapist can often help with this process.