Pay back the money!!!

I have been a cell C customer for many years, and all I expected from them is decent service, especially towards long standing customers. My contract ended on the 27th of April. I phoned in on the 3 of April, as it was Easter from the 27th of March to the 2nd of April. I notified them of cancelling and they said ya sure no problem sir and even offered me a month on month contract for R30 which includes airtime and data. At the end of May I phoned to confirm all this because I was not confident that they could actually pull of a simple cancellation. True as bob, 'oh sir, the contract was not cancelled'. Which means the airtime i used in May which I thought I was paying R30, they are now going to charge me the full contract price for the month of may, it is blatantly dishonest, *********, unprofessional and ********. I had a contract, I cancelled, they deceived me to get more money out of me.