Check phone number region Việt Nam

Paste code to function.php in Child theme

function custom_filter_wpcf7_is_tel( $result, $tel ) { 
  $result = preg_match( '/^(032|033|034|035|036|037|038|039|086|096|097|098|081|082|083|084|085|088|091|094|056|058|092|070|076|077|078|079|089|090|093|099|059)+([0-9]{7})$/;', $tel ); 
  return $result; 
} 
add_filter( 'wpcf7_is_tel', 'custom_filter_wpcf7_is_tel', 10, 2 );