Being comfortable with social interactions is a skill you can build up!
You build it up through practice!
You chat with anyone you can!
I don't mean online ;) - LIVE!
You are waiting somewhere, start a conversation with a stranger.
Do that frequently! 10 times/day!
This will boost your confidence next time you pick up the phone.
Why? Because your social skills are already active.
It is simply a matter of transfering them and using them within a selling context.