Spend limit increased

I have a contract with Telkom and i have a limit on my contract, to my surprise i was able to make calls even after my minutes had finished, at 5.11pm today i called the call centre to enquire and was told that the limit was increased but the agent did not explain how and why this was done,she said it was not their department who did this, i did not understand what this 'increase of spend limit'was because i usually call in and consent for this to be done and not for it to be automatic! If i pay more whose fault will that be?? This limit should not have been done without my consent especially with calls since i will be billed for that