Nervous about talking on phone?

I do some voluntary work and my boss was asking if I would be willing to talk on the phone, but i wasn't too keen, so it didn't happen. Now that I think about it, I know it's not good for my job prospects if I'm not keen on talking on the phone. The main reason is I feel as if Im not very good at it and I'd get nervous in case a customer asked me something which I found difficult to answer. Any ideas as to how I can overcome my nervousness regarding tralking on the phone? Should I just go for it next time i do the voluntary work?