Help with my anxiety attack?

Some guy is supposed to come over to our house and look at a car that my dad has up for sale. My dad just decided that he was going to leave and go do God knows what. So he gave me his cell phone that the guy is supposed to call him on and told me to tell him to come over, and give him directions to our house. I really am freaking out now. I have severe social anxiety. Talking on the phone to a relative can send my into a panic attack, much less talking to a stranger. I am really starting to freak out. I know that as soon as that phone rings I will go into a full on anxiety attack and probably not even be able to talk. I'm already getting shaky, nauseated and starting to cry just thinking about it. How can I get control over my anxiety attack? If I don't talk to the guy my dad will be incredibly pissed. But I don't think I can do this. Please help.