I used to be in the same boat when I was self employed. My solution was to rely heavily on email (when possible). In cases where a phone call was necessary, I would mentally plot out the first 30 or so seconds. I found that 30 second prep was enough to get me over the hump of actually making the call. On those items where you are not knowledgeable, you can say "I will look into that and get back to you". I found 99% of people were fine with that answer.

edit: Remember, nobody likes to talking to random people on the phone. The other end of your conversation is probably feeling very similar to you.