Do you have some starting trouble to get your crush's phone number? Then below are some ways to ask.
If you are confident, then ask mobile number directly by simply saying "What is your mobile number?" or "Can i get your number?". But be prepared for the question "Why?"
Give your number first
If you are nervous to get your crush's phone number, then give your number by simply writing on a paper or let her feed on her phone.
You type for your crush
If your crush is using her phone, then tell that you will feed your number.
If you are chatting on facebook or any other chatting sites, then say "As am going to a place where there is no good coverage of internet, can you give me your mobile number?"
Make your crush happy
If your crush recently achieved something or tried something new on that day, just compliment on that and have a bit conversation. At the end just ask for their number.
What if she says "No"
If your crush is not interested to exchange mobile number with you, then don't compel them. You should either move on or wait for another chance.
Get the number indirectly
If you are not much confident or shy to ask her number directly, then get it from a mutual friend of you both.
Suppose you are a student and your class/school mate is your crush, then get number from a mutual friend who has your crush's number. You should have a non-personal reason before making a call like you got her lost notebook or something and you wanted to return to her. Or you are a member of some school club or committee and you wanted to invite her to join or participate.
Suppose you are working in an office and your colleague is your crush, then you call your crush for official purpose. Then slowly change the track so that she won't understand you are trying to creep.
Suppose you and your crush share some common interests, you can tell that there is some event happening near and ask if they would like to join you and get number if they seem interested.
If you need help in something or your crush's seems helpless, share your number and explain your point.
Suppose your crush lost something precious, offer help and get mobile number when everything is normal.
In case you are in trouble with something, talk with your crush and at the end say "I feel better now, i would like to have your number"