How to ask for phone number?

Know that people don't want to give their personal information.
First step is to communicate with your partner(I suggest minimum a week).
Don't hurry, be patient.
When chatting try to looks like that you are interested in what he/she
says.Mention things that he/she has been talking about.
If the other person refuse to give you the number- don't stop to chat.
May be you will get it in following conversation.

First ask for e-mail - it protect personal information.

It is a good idea to give your phone.