mapdev\FacebookMessenger\MessengerUtils::checkPhoneNumber PHP Method

    public static function checkPhoneNumber($string)
    {
        if (!starts_with($string, '+')) {
            throw CouldNotCreateMessage::invalidPhoneNumber();
        }
        return $string;
    }

 public function phone_number($number)
 {
     MessengerUtils::checkPhoneNumber($number);
     $this->type(ButtonType::PhoneNumber);
     $this->payload = $number;
     return $this;
 }