Because human brains aren't supposed to be able to do emotional and mental parkour like completely redirecting our focus on a new topic in under a second without prior warning.

For me personally, if it's a call I expect, it drops the anxiety levels by considerable amount. Even better, if I can prepare a plan/scenario for the call, and take notes during it.

Of course, if it's someone from my close family or friends, that also helps. But unexpected calls from unknown numbers (or known, but from like work or distant acquaintances) freak me out.

For me, I think it's the fact that I have to prepare for both a social interaction and a monologue depending on whether they answer or not. As someone with mild social anxiety, the uncertainty and the fact that I am unequivocally initiating the interaction messes with a lot of the ways I would cope with joining a normal social interaction and throws me off my game

I got a lot better at phone calls when I worked on trauma therapy. I've always had issues asserting myself and phone calls are sort of a form of saying "hey!! Quit what you're doing and pay attention to me because I have something important!" which isn't something I was used to doing or asking for nor did I even think anything I did was important