Vodacom out of data bundle charges

I am tired of Vodacom using my airtime automatically every time my data bundle has depleted! They send the warning sms way too late. By the time it arrives everything is sitting at zero. It should not just use airtime when the data bundle is depleted. Learn from your competitors! At least one of them know what is the proper way to design a billing system cause your testing team is definitely not doing they jobs if they can't even pick up this fatal flaw.