3 opportunities as I see it:
1) You write an email to your boss right away where you explain the situation. Take your time. Write it well. Just explain that you are happy working there (use some other words, it should sound professional), and you fully understand that it irritates your boss when you are not able to answer the phone. Then explain that even if you give yourself maximum at your job, there are some things you are just not able to do yet, due to some complications regarding social fobia, e.g. taking the phone. but you are working on it every day, and has set up a meating with a therapist to solve the problem, because you understand how important it is for your boss that you talk on the phone to finish the tasks faster.
Pros: You tell it as it is. If your boss is a good man, he will understand you, and even ask how he can help to solve this issue. He will be very glad that you told him that it was the reason, and that it was not an ignorance of any kind.
Cons: I say it as it is, if there have been some mistakes in the way you have done the jobs, he will think, well, thats an other good reason to fire her. But, IF you have done your job very well before, no risk of getting fired.
2) You just take the phone and talk. Sometimes in life, we are more capable than we think. It takes small steps in life to acomplish greater things. Why not start practicing today? Use relaxing music on youtube to help you

Pros: If I have not taken smalls steps in my life where I had anxiety, I would not have time to write this post and trying to help other people. Today, because I took this steps, I have what I need in my life, and even the time to help other people to acomplish what they want

It all started with one small step, where I said to myself, life is about enjoying everything. Enjoyng taking this "scary" small steps, because you develop as a person

Deep inside you want this development too

Cons: You cant live your life on a lie. Sometimes you just have to tell people the truth and then, you learn that its okay. People accept you as you are. And when you learn that, your can do anything you want in your life. Anything

3) Getting your therapist to write your boss a message.
Pros: The boss will know the truth, and will not be irritated of what he thought was your "i dont care with my job, therefore i dont take the calls"
Cons: Be a strong woman. Write that email yourself. It shows that you accept yourself as you are. Its a very strong sign.
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Take a walk. Take a bath. Relax. Then take the decision. In the end, it dont matter which one you take. Just take one. We are only stardust in the entire universe. What the universe want will happen, so stop worrying about "well if i take 1,2 or 3, it will be a mistake". There are no mistakes in life. Everything has its own order. You get fired? Is it bad? No. You will learn more about yourself than ever. It will be the point where you understand the entire meaning of life. Its ok to be who you are. You dont get fired and he accept it? What a beautful feeling. One more irrelevant thought in our mind. Do you know how many irrelevant thoughts a normal person think during the day? 90%

Source: Eckhart Tolle. Where is your 90? Is getting fired part of the 90%? Stop worring and start living? Just be yourself and take that decision that is closest to your innner hart, to your inner beauty and be proud of yourself, because, it IS so beautiful that even with this diagnosis you help other people by transcribing things, just amazing - i love you