You walked up to her for a reason. You were clearly drawn to her. You may not know her, but that is what chatting with each other over the phone is for.
Despite what your friends told you, if the prospect of going out with a stranger was creepy to this particular girl, she wouldn’t have given you her number to begin with. She was interested in getting to know you as well.
Your next move should be to use her number to contact her and engage in conversation. If you like what you hear, you could then proceed to ask her out.
You may be a little bit different from your friends in that you were able to walk up to this girl and ask for her number.
Your friends most likely never do that. They would much rather play it safe and stick to talking to girls that they know.
If this isn’t who you are, this may be an occasion to listen to your instincts as opposed to listening to your friends.
All relationships start from a point of not knowing each other at first. You made a move on this girl for a reason. It would be best to follow it through and see where it all leads.