This used to happen to me as well. It's a bit embarrassing to admit, but as a teenager I would get very anxious at even the idea of calling someone I didn't know. The feeling vanishes once you make enough calls; as simple as it sounds, the key is to treat phone calls as regular conversations. If anything, calls should be less stressful than everyday conversation, since you don't need to worry about looking engaged or attentive and you can't see who you're conversing with.
