function synchronizePrepaidBalance()
    {
        if (!$this->getCustomerId()) {
            throw new BalanceException("Customer Id is undefined");
        }

        $response = $this->call('getprepaidbalance', array(
            "customer_id" => $this->getCustomerId()
        ));

        $this->setPrepaidBalance($response['prepaid_balance']);
        return $this;
    }