Now, I know, you’re thinking to yourself that this is so obvious! But it is in fact the best approach. Instead of “getting” her to give you her phone number, just be honest and ask for it.
But here’s what's important – don’t do this online. There are probably a million other guys doing the same thing. Instead, walk over to her and express how you feel. Then, politely ask if she’d be okay sharing her number with you.
But please know that this method works when you already know the girl to some degree. If you’re strangers, she might not be entirely comfortable sharing her phone number. The two variables you need to nail for this to work are trust and interesting conversations.
If she actually wants to share her number, she will. When you do ask her for her number, be VERY casual about it and slip the question in-between sentences.
If she has already said she will give you her number, give it some time before you sit and sulk with a pack of tissues.
Don’t keep badgering her about giving you her number, that makes you repulsive. Don’t be clingy, and give her some time.