You can pay for several months in advance (you don’t have to use up the funds the same month.) Funds will stay in your account for at least 12 months after the last time a vehicle was activated, i.e. if you have USD 100 on your TaxiCaller account and your last vehicle activation was in August 2018 and you haven’t activated a vehicle since then, the funds will stay on the account until August 2019. If you activate at least one vehicle no later than August 2019 then the funds will be kept on the account for another 12 months after the latest vehicle activation.