Kevin Drum collects a bunch of bloggers saying how much they hate using the phone. Count me in their number, though until now I didn't realize I had company. I don't mind talking in person, and I'm a big fan of AIM, but I loathe calling folks on the telephone.
I think the problem is that phones imply intentionality. If you're IMing someone, it's very spontaneous -- "oh, you're online, so why don't we chat?" Even email is very casual. But you know exactly what you're doing when you call someone on the phone. You know you want to speak to the receive, and they know it, which for introverts makes every phone call like asking your crush on a date. You're always afraid they'll wonder why this loser is bothering them.