Does a usage limit @mtn do what it is supposed to?

I have been an mtn customer for many years and cant even begin to imgaine how much they have made from me as their customer. Last year I took out an upgrade on one of the two lines that I have with them; it was done through one of their mtn direct call centres. The upgrade went well and during that process, i specifically requested that a usage limit of zero should be set on this line, I specified this more than once and i was assured that indeed the limit has been set to zero. <br> <br> I did that to avoid getting a huge bill by using more than what i had on the device. But i was shocked by a debit order from mtn on the 25th of February, which was R1800 above what I am supposed to pay and when i called in to enquire i was told that was due to the fact that i used up 2gigs of data above the monthly allocated 500mb. This was a huge surprise seeing that I had requested my usage limit be set to zero. This all seems too convenient for mtn, they would seem to have alot to benefit from them not doing what has been requested. I am very disappointed in what has happened and I cant say I have much faith left in the company.