D_Hahn

Solved with textarea ‘tellus’

function custom_textarea_validation_filter($result, $tag) {  
$type = $tag['type'];
$name = $tag['name'];
if($name == 'tellus') {
$value = $_POST[$name];
$Match_all = "/^8(\d{10})/";
if(preg_match($Match_all,$value)){

    $result->invalidate( $tag, "11-digit numbers are not allowed !" );
                }
}
return $result;
}
add_filter('wpcf7_validate_textarea','custom_textarea_validation_filter', 10, 2);
add_filter('wpcf7_validate_textarea*', 'custom_textarea_validation_filter', 10, 2);

Fixed.

For some reason, my theme was missing the wp_head hook. Adding it made it work. Thanks for your response.

<?php
do_action(‘wp_head’);
?>