Since those errors occurred at lines 10 and 11, they must have been placed before the html code, they are undefined on first load as there is no post yet. So you should always check for the existence of any post using
isset()
before accessing it, e.g.
if(isset($_POST['number']) && isset($_POST['message'])){
$number1 = $_POST['number'];
$message = $_POST['message'];
$message = urlencode($message);
}