Contract cancelled in Jan 2017 but still receiving invoices

I cannot begin to describe the lack of customer service and frustration that I have experienced since cancelling my cell phone contract with MTN on 26 January 2017. I terminated by cell phone contract (early termination) with the help of Mr Mario Breedt, MTN employee at Woodlands Blvd, Pretoria, on 26 January 2017. This was confirmed in store and a full payment was made as advised by Mario after phoning the MTN retentions team. I relocated overseas on 28 Jan 2017. I subsequently received an invoice from MTN in February 2017 indicating that I still owed the approximately R1,000. My sister, who resides in Pretoria visited the Woodlands Blvd MTN store and settled this amount. I have however now noted that my current statement shows that I owe MTN an amount of R3,465.48. How is this even possible if my contract has been CANCELLED???? I have spoken to call center agent Primrose Mvubu on 27 Sept. 2017 (Ref nr ********** 20) who confirmed that the notes on the system indicate that I requested and paid for an early contract termination in January 2017. I urgently require someone to correct this as MTN is currently committing ***** by intentionally charging me for a service they are not authorised to or have been providing for the last 8 months! The retentions department is never available and they do not return emails!