how to compare my contact with the contacts upload

Question

Firstly I would like to show you my Firebase database tree

-Users
   |
   ----959670000
   |      |
   |      ------name
   |
   + ----750890000
   +------859200000

I had a root node with child as a user. whenever new user registers it got new Unique Id... and the unique Id is the phone number. Now In My Android Application I retrieve all my contacts i.e. phone number present in my mobile number. All I want is to compare Each phone number in my contact to the phone number present in the firebase database so that I would get to know which user in my contact had got registered.

All I am trying to ask are the ways to achieve the bold part in my question please Help me....I am very upset since the past 4 days and trying to achieve. Anyways thanx in Advance....:-)