public function credit(): ?array
    {
        $this->checkConfig();

        $query = http_build_query([
            'userkey' => $this->userkey,
            'passkey' => $this->passkey,
        ]);

        $response = Request::get($this->baseUrl.'/smsapibalance.php?'.$query);

        $xml = simplexml_load_string($response->body);
        $body = json_decode(json_encode($xml), true);

        return [
            'code'    => $response->code,
            'message' => ($response->code == 200) ? 'OK' : $body['message']['text'] ?? '',
            'data'    => $body,
        ];
    }