We are using different ISA providers for each of our target countries, so the mechanisms of the ISAs are slightly different. In some countries you will have a grace period (of upto 6 months from your start date). In others this has not been possible. Please refer to your agreement to find out.
Irrespective of the timeframe, you will not have to pay unless you are paid more than your payment threshold - which is the minimum amount you need to earn. Equally you ISA expired automatically if you do not get paid more than the payment threshold for 60 months.