You can set your account to automatically purchase World Credits by logging in to your Freephoneline account, clicking on "Online Store" and changing your "Auto-renew Status" to "Enabled". Auto-renew automatically credits your account with your previously purchased World Credit amount when your balance drops below $1.
Please note that call costing happens per minute and auto-renew is not immediate so auto-renew does not top up your balance while on a call. Your call will end if your World Credit balance has insufficient funds. Also, auto renew is currently a nightly process so you will see the auto renew amount applied to your account the next day.