CALL LIMIT EXCEEDED

Hi I got a massive problem, I have a phone contract with vodacom actually 3 contracts but this specific one has seem to have become the problem I have a set call limit on my phone of R50 and occasionally I now and then reach my limit and my phone automatically gets softlocked I then I have to call 111 to extend the limit usually by additional R50 or more I have been doing this for about 2-3 years no problems when I effentually see that I go past my affordability I don't extend the limit I then leave the phone soft locked and wait until new month for my contract data to replenish like every other month and when needed I'll extend the limit, but for some reason January month I receive an invoice for an amount of R7050 on data charges only but my limit was set to R450 and on the last day 30th of Dec I got my 3 sms notfing me my limit was 50% then 90% then soft locked I then left it once again due to affordability waiting for the new month I call the center they explain this happens you must pay. I have been 8 years loyal customer with Vodacom no problems with payment and I feel this is not acceptable I pay almost R20k a year on a phone bill. Now the limit doesn't work over Dec???