Bad service

I had an airtime contract with one 2 one since 2011. This a lady from one 2 one called me to tell me that my contract will be coming to an end on the 11th of June 2015. She wanted to know if I'm going to renew my contract or not. I said no I'm not going to renew it. I requested that she cancel my contract. She gave me to numbers to port back. She said I must port back a week before the expiry date. In May 2015 I called one 2 one to give them a notice they said I must wait until my contract expires. Now since I have successfully ported back my number and my number is cancelled on MTN, I am still billed for the air I don't get. I called to enquire why I'm I still be billed. I was told that I did not give them notice which does not make sense because the first time they called to renew the contract that's when I told them that I'm not continuing with the contract. Now I am paying for the airtime I don't get. One 2 one you are ripping of innocent people.