Phone line cancelled however still being billed

Dear Sir / Madam<br> <br> My mum's Telkom home line was cancelled in January 2015 with the last payment occurring in February 2015 however each month she is still being billed for the disconnected line. The reference for the disconnection was ***A. Kindly can you pass a credit for all the months you billed her for a disconnected Telkom line.