If you don't want to run out of credit, or for your credit not to fall below 30 minutes, you can activate the Auto-recharge.
How to activate the Auto-recharge
Go to My Account > Top-up & Plans
After selecting the number of hours you'd like to purchase, the Auto-recharge option will appear
Click on 'Enable Auto-recharge'
You'll be lead to the check-out page, and you'll notice that 'Auto-recharge' will appear in the Order Summary
After you hit 'Confirm', every time your credit falls below 30 minutes, the system will automatically purchase more credit for you
How to deactivate the Auto-recharge
Go to My Account > Settings
Select 'Plan', on the right side of the Settings page
Click on the 'Disable Auto-recharge' button and refresh the page