My contract that was supposed to be cancelled and expired im still being charged for

my contract expired 31/08/17 which I took for internet called and cancelled it, that's was not billed for the passed two months now I get an sms saying I'm on errors with my account, thinking its the one I took this year 31/07/17. Apparently I owe the one that is expired, this no fair on all levels because I called to cancel my contract they have the recordings and the conversations, this that rose to R400 I have to pay because someone forgot to cancel my contract. Called the call centre they say they can't help and transferred from on person to another .