I have had endless debates with Vodacom on this very matter.
The package I am on offers a so-called carry over for one month, which means unused data in September should be available in October to augment the October data. The "carry over" then expires at the end of the next month.
However one hardly ever benefits from this carry over, because it only becomes active AFTER the current month's data is used up. Effectively it means most of the time for me, I never get to use that data.
They should be using the FIFO principal but are not, where last months data is first worked off before this months data starts being used up. Vodacom have not once replied to any of my e-mails on the matter not in 5 years have they.
The next gripe I have is Vodacom offers a Night Owl allowance that kicks in at midnight. BUT to access that data one has to remember to log off at midnight (on my tablet, that means powering down the device), before data usage will now count down in the so -called "night owl" allocation. Again Vodacom has never replied to an e-mail on the matter, nor have they been able to explain it when demonstrating these issues to Vodacom personnel.
So, One of the very first steps in all of this debate about lowering the cost of data, is to STOP expiring data at all, and sort out the systems so that the count down of data works according to the FIFO principal, as well as gets the systems to "switch" from normal data to the night owl allowances without requiring customers to log off or worse power down their devices.