public function addPhonesWithVariables($bookID, array $phonesWithVariables)
    {
        if (empty($bookID)) {
            return $this->handleError('Empty book id');
        }

        $data = [
            'addressBookId' => $bookID,
            'phones' => json_encode($phonesWithVariables)
        ];

        $requestResult = $this->sendRequest('/sms/numbers/variables', 'POST', $data);

        return $this->handleResult($requestResult);
    }