Hey, I seem to be kind of in the same situation as you. I got a girl's number yesterday and still deciding whether to call or not. But after reading all of the posts on this thread, I will definitely call tomorrow. This is how I see it now...
1. If you call and screw up, then you will never see that other person again. So you don't have to worry about how you came off because you won't have to deal with them. But the best part is, you got to practice a phone conversation and you will know what not to do for the next time the chance comes around.
2. If you end up getting to hang out with them, than whatever happens during the hang out is fine. Again, if you screw up, then you won't see them again. If you are successful, then that's cool. It will be a great experience and you can definitely have learn from you mistakes and successes. Plus, you have a great story for a next conversation.
3. Finally, if in the end they just want to be friends, then cool. It would be great practice with talking to girls. How many friends that are girls can you say that you actually have? Aren't even just friends what a lot of SAers are looking for anyway.
Now, being that I have SA also, I know that all of this is easier said than done. But I guess the only way to really get over calling and hanging out is to actually call and hang out. The first time might be extremely hard but it should get easier just like everything else does with practice. Anyway, hopes this helps not just the poster but anyone that is contemplating making a move on something, anything. I hope this post makes sense. I think I just convinced myself to call the girl...