Dont ask directly for her phone number, have a real "long" conversation first, then tell her that you enjoyed talking to her, if she likes you she will give you her phone number.

greeting her firstly than you ask her about something . for example to show you a direction or anything after that thank her ask her about her name and tell her your name you will both make a short conversation than you can ask her about her phone number

say a girl that you love good news she will as what?
u reply that .. give me a surprise by giving your number ?
it will be a real good news for me lol