Billed for Cancelled service.

I am once again being hounded by a collection agency for a service I no longer have. Nudebt have emailed me outstanding accounts for Telkom dating back to January 2016. These accounts are for line rental for a place I moved out of in December 2015. As such I cancelled the line in store. If there was a reference number given to me it has long since been miss-placed. How can I be billed for a service at an address I no longer occupy?