Thanks for you post, and welcome to the forum! I trust you will find this a useful, informative place to be.
Black and white makes some good suggestions - I'd recommend checking out the articles and consider the downloads.
My first thoughts are - you've only been in the job 5 days - give yourself a break! You were recruited to the position as you have the right knowledge skills and attitude and are able to learn any new skills.
I am wondering if my feelings of telephone phobia, social inadequacy and lack of self confidence will continue to hamper me throughout my life and affect every job I do.
Before you write yourself off as a lost cause, I think you need to look "outside" of yourself for some solutions. What I suspect is that you are lacking a few skills, or need to brush up on a few others and could benefit from a few tools of the trade.
In, for example, telephone fundraising, callers are given a "script" to help them through the call. This usually consists of some kind of opening to build rapport quickly, a sales pitch including objection responses, and a close to make the sale. This is just like an actor learning his lines - compare that with doing improvisation which is much more challenging, more risky and very stressful for novice actors. Can you write such a call script, or ask your employers to help you with one?
There are 101 ideas for how to build your confidence at this Uncommon Knowledge site
- just sign up for the free email course.
In summary, think about this like training for a race - you need to get some skills and also do some practice. This is the only way to improve - you will not win the race by wishful thinking!
I hope this is a useful start - you will improve!