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/**
* Process the checkout
**/
add_action('woocommerce_checkout_process', 'my_custom_checkout_field_process');

function my_custom_checkout_field_process() {
  global $woocommerce;

  if ( ! (preg_match('/^[+]{0,1}[0-9]{10,14}$/D', $_POST['billing_phone'] ) ) ){
    wc_add_notice( "<strong>Telefón</strong> musí obsahovať minimálne 10 čísiel." ,'error' );
  }
}