You must have sufficient credit in order to subscribe to the service and must maintain the minimum credit required to access the Service and for the recurring monthly fee. If you do not have sufficient credit to renew your monthly subscription at the end of each calendar month, you would not be able to use the service.
The system would daily try to renew your monthly subscription for 15 more days (if successful, you would continue to enjoy the service for the following month). During this period, you can top-up your balance, and retry the service whereby you would get the option to renew the subscription manually.