I know exactly how you feel. To this day, I have the worst difficulties making inquiries by phone, especially when I have to phone an unknown person. Writing down what I'm planning to say hardly ever works, since as soon as things don't go the way I was planning I get completely disorientated (it's enough for me to get a voice I wasn't expecting -a man instead of a woman, or vice versa- to put me off balance). Calling a helpdesk is a nightmare: I usually have to go and lie down afterwards, until I stop shaking at least. And while it's bad enough when I'm alone in a room, it's even worse when somebody is there and can overhear me! Then I just freeze completely. It took me a long time to make people understand how scary calling people by phone was to me.
I got slightly better: I used to have these problems with everyone. Now, I can more or less call friends and family without breaking too much of a sweat (it's still difficult, especially if I have to make a "social" call, but it's not as bad as it used to be). It's just calling unknown people that really frightens me. I can do it if I really need to, but it's not a nice experience and something I want to do as rarely as possible. Thank god for e-mail and SMS!
That said, I wouldn't worry too much about their reaction: they will probably think you had a bad connection or something. Usually that was what my friends asked me when calling them went wrong.