Exchange your number to get a girls number
Girls will trust you only when you give them something to rely on. So, make some exchange with them. You can give your address or your contact number. Most effective is giving your phone number. Then you can ask for her number in exchange. So, when you call her she will be able to recognize you and will feel comfortable to talk with you. So, stop whining about how to get a girls number. Just go for it. Do not make any rough comment about the girl and her interests. Express your thoughts and ask her phone number. She will give it surely because they are also human and they have mind also.