spend limit

I have a cellphone top up contract with Telkom. Usually if I have finished my account I will call their centre and ask for a top. Yesterday I found out that Telkom had increased my spend limit by R500 WITHOUT INFORMING ME. I NEVER RECEIVED ANY SMS OR CALL INFORMING ME ABOUT THIS. For the next two months I will be paying more than a R1000 whereas my budget for the phone bill is R528. I feel abused and ambushed as a customer. It is because of situations like this that make me want to warn other customers. When signing the contract, I knew what I could afford and went for an option that will keep my bank balance healthy at all times. This is totally unacceptable and will take further action if the 'R500' is deducted from my account month end. I never asked for it. There was no agreement between me and the service provider regarding this.