Credit Limit R2500 but received a bill of R8000???

In April 2015 app**** for an additional contract, it was approved however my credit limit was set on approx. R550. credit limit was then increased to approx. R2550. Line was suspended after 2 days of having the new phone due to credit limit reached which was fine because I reached my limit of R2550, payment was made of R1000 to open the account again and within approx a week after it was opened MTN sent me a bill of close to R8000. What is the use of a credit limit then on your system because surely there was a system issue as to why my line was not suspended when reached the R2550 again. Even your credit controllers cannot explain to me why this happened? To my understanding, a credit limit is set so you cannot overspend, if you go to Edgars and your credit limit is R10 000 they will not allow you to buy for R50 000 surely. Sort out my account please and your systems