I am trying to rewrite urls in WordPress, but its not working

You can try

add_action( 'init', 'wpse26388_rewrites_init', 10, 0);
function wpse26388_rewrites_init(){
    add_rewrite_rule(
        '^us-dot-numbers/([A-Za-z0-9-]+)/?',
        'index.php?page_id=642&dotNumber=$matches[1]',
        'top' );
}

add_filter( 'query_vars', 'wpse26388_query_vars' );
function wpse26388_query_vars( $query_vars ){
    $query_vars[] = 'dotNumber';
    return $query_vars;
}

Remember to flush_rewrite_rules();