Tricky SMS forcing customers into paying

Since taking out a contract with Telkom in December I've always been given 100 mins and charged for whatever i increase my spend-limit by thereafter (upon my request). I have recieved my statement for the month of August stating that Telkom is charging me an additional R500 over and above what i have requested. This follows an SMS sent by Telkom (which i did not read) asking customers to reply NO to a fixed limit of R500 or they would add the R500 if people rep**** YES or DID NOT reply