On your account billing page, you can set your account to automatically recharge your balance when it falls below a chosen threshold.
To set up your recharge trigger, look under the AUTO-RECHARGE section and click the Enable button. On the Auto Recharge page, you can specify the trigger threshold amount and the amount that Twilio should recharge the balance to.
While your recharge trigger is enabled, anytime your account falls below the trigger amount, Twilio will automatically charge your card to bring your balance up to the amount you set. Note the amount charged to your card will be the difference between the recharge target balance you've set and your current account balance. This is not always an even number and may vary depending on your usage.
It is possible to also set a notification trigger that will just email you when your balance falls below a chosen threshold. If you'd like to enable this please contact Support.