Every invoice contains the call history from the 16 of the previous month to 15 of the following month.
The first invoice will show the subscription charges for the period between the activation date and the end of the first full invoicing cycle.
In short, each month you pay for the calls/SMS/surfing time consumed during the previous month plus the subscription costs for the next month.
If you have cancelled your subscription during the previous month you will have overpaid in subscription costs.
This will be offset via a credit applied to the next invoice (reverse rental). If you want to be reimbursed directly, you just have to issue a request after you receive the negative invoice (provided you are not owing).