The reason why your account displays a negative balance is because you’re being billed for usage in arrears. Your usage of the system is registered in real time. Funds are deducted from your account balance as you activate a vehicle, and when you make a payment, it will be displayed in the billing overview and your account balance will be updated accordingly.
Your billing overview can be found here: https://admin.taxicaller.net/admin/billing/overview.php