Never modify the WordPress core table schemas, such as wp_users etc under any circumstances.
The next time WordPress changes its tables, your custom columns will be destroyed, and all the data will be deleted. This is unavoidable.
Instead, use the User Meta API and table:
Retrieving a user meta:
$phone = get_user_meta( $user_id, 'phone_number', true );
Adding/updating a user meta:
update_user_meta( $user_id, 'phone_number', '0123-456-7890' );
Where $user_id is the ID of the user
Never modify the tables, store the data in the meta tables instead using the functions.