Cancelled service but still invoiced

Cancelled our line at the beginning of September and was given confirmation that the line had been cancelled effective 8 October 2015. I now receive an invoice for the period 9 October to 8 September. I don't even live at that place of residence anymore. I received a reference number as confirmation. I am really not impressed as one sits on hold for at least half an hour in order to speak to someone and then a further 20 minutes to process the cancellation!