Enabling Auto-Refill is the best way to make sure you don't experience any balance related service interruptions. Follow these steps to set it up and forget about having to ever manually reload your account.
1) In your Dashboard navigate to the Billing section.
2) Once in the Billing section make sure there is a valid payment method on file in the Payment Methods card.
3) Enable Auto-Refill by toggling the button in the Auto Refill card.
The settings in the Auto-Refill section allow you to change how much to add to your account and when to add it. This varies from use case to use case, but a popular choice is to add enough balance to cover your organization for a full month to reduce the amount of charges to your credit card to about one a month.
4) Hit the Apply button.
Enabling Auto-Refill on activation
Alternatively you can also enable Auto-Refill in Step 3, Add Payment, of the SIM activation process.
Note: Billing settings are set at the organization level