Billed after service cancellation

I cancelled my account at the end of last year and my line was deactivated on the 26th of December 2017. I received an invoice for January 2018 and was subsequently debited. I have just received an invoice for February... despite numerous efforts to correct the January error! I was starting to appreciate Telkom as a substantial service provider :( An example: telkom double debited me last year; I alerted them and a correction was made within 48 hours! What’s with the fluctuating service! Please stop taking money for a service you are not providing... I think that’s called theft :(