Well, tell them the main reason why you want a phone- if they still say no then there's nothing much you can do about it but drop hints that you really want one. Don't constantly bug them about it, that won't get you anywhere... Write a letter to them, they'll think its really mature. : D

Why don't you find a ways to own one ? I guest you can get what you want if labor for it.

How old are you? Even if you work on your own to have a phone for yourself, your parents still confiscate it since they don't want you to have one. I think you're still young, for now just follow the demand of your parents.