public function addContact($phoneNumber, $firstName, $lastName)
    {
        $phoneNumber = preg_replace('%[^0-9]%', '', (string) $phoneNumber);
        if (empty($phoneNumber)) {
            return false;
        }

        return $this->exec('add_contact ' . $phoneNumber . ' ' . $this->escapeStringArgument($firstName)
            . ' ' . $this->escapeStringArgument($lastName));
    }