Balance
The left menu shows the current credit balance and roughly how many minutes it buys. Configuration → Billing shows the balance, recent activity and the payment method.
Top up
The page also shows the current plan and the per-minute rate your calls are billed at. Press Add Credits and pick an amount to top up. Turn on automatic top-up to add a fixed amount when the balance falls below a threshold, with a monthly cap.
When credits run out, new calls and campaigns are blocked until the balance is topped up. Set a credit threshold alert under Notifications.
Read the balance over the API: Get billing summary.