How to get all child's data in firebase database?

First retrieve your users datasnapshot.

//Get datasnapshot at your "users" root node
    DatabaseReference ref = FirebaseDatabase.getInstance().getReference().child("users");
    ref.addListenerForSingleValueEvent(
            new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {
                    //Get map of users in datasnapshot
                    collectPhoneNumbers((Map<String,Object>) dataSnapshot.getValue());
                }

                @Override
                public void onCancelled(DatabaseError databaseError) {
                    //handle databaseError
                }
            });

Then loop through users, accessing their map and collecting the phone field.

private void collectPhoneNumbers(Map<String,Object> users) {

    ArrayList<Long> phoneNumbers = new ArrayList<>();

    //iterate through each user, ignoring their UID
    for (Map.Entry<String, Object> entry : users.entrySet()){

        //Get user map
        Map singleUser = (Map) entry.getValue();
        //Get phone field and append to list
        phoneNumbers.add((Long) singleUser.get("phone"));
    }

    System.out.println(phoneNumbers.toString());
}

This listener will only retrieve the datasnapshot when explicitly called. Consider storing a list of numbers under an "allPhoneNumbers" node in addition to your users node. This will make your datasnapshots lighter and easier to process when collecting all numbers. If you have say hundreds of users, the "users" datasnapshot will be way too big and the "allPhoneNumbers" list will be far more efficient.

The above code was tested on your sample database and does work. However, your phone field may need to be casted to String or int depending on your user's phone instance field's type.

      DatabaseReference ref1= FirebaseDatabase.getInstance().getReference();
      DatabaseReference ref2,ref3,ref4;    
       ref2 = ref1.child("User");


       ref2.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {

            Userlist = new ArrayList<String>();                
            // Result will be holded Here
            for (DataSnapshot dsp : dataSnapshot.getChildren()) {
                  Userlist.add(String.valueOf(dsp.geValue())); //add result into array list

                }
    /* userlist will store all values of users, then point to every userlist item 
    and get mobile numbers*/