contract amount changed without notify me

I have a R275 top-up contract which I was paying R188.43 per month because of a special Cellucity was having. I noticed in the week that since Sept 2015 my debited order changed to R294.50 without notify me. I immediately called the call centre and asked why the increase to which she responded that my 24 month contract expired in Aug 2015 and the discount I received all the time fell away. I understand that part but the problem that i have was the least Vodacom could have done was to inform me that my contract has expired and that I can no longer receive the the approx R100 discount. They shouldnt just automatically increase my debit order. Luckily I had money in the account to compensate for the extra R100 p/m that got deducted else I would have gotten a bad name if I didn't have enough funds in my bank account. PLEASE INFORM CUSTOMERS REGARDING CHANGE OF DEBIT ORDER AMOUNTS WHEN THE CUSTOMERS CONTRACT EXPIRES IN THE FUTURE!!! from very irate customer :(