I think the situation is more like this:
Extraverted children and teenagers are normally so compelled to get out there and interact with people that they've naturally developed the confidence/outgoingness/whatever over time and practice. Being 'outgoing' is as much a skill as anything else, trust me. It requires practice, practice, a bit of failure and a lot of acting. And more practice. So by the time they're adults, they've embarrassed themselves enough times that nothing much phases them anymore. :P
Introverts, on the other hand, having much less compulsion to go out and make a fool of themselves, sometimes miss out on getting over the embarrassment when they're very young and impervious to other people's opinions. So then it may get awkward in the teen / young adult years. Hence the (false) association between introversion and shyness.
For the record, though, until recently I had to write down what I needed to ask the shop assistant over the phone before I called them. I needed it to center me and keep me focused, clear, and concise. Phones used to scare me. But then I got a job where answering the phone and dealing with people vocally only (not my preferred communication style!) was a major part, and so I got over it. I'm much better now.