Account paid up but contract not cancelled

I've had 2 attempts to cancel a contract I now longer use. <br> Still I get an invoice for the month ending May and guess what, the amount is the one that I've paid up after I was assured that this is now sorted.<br> Can I please get someone competent with the necessary authority that will phone me to resolve this issue!! I'm tired of phoning back and whoever I speak to cannot finalise this!!<br> PLEASE CELLC!!