Helkom overcharge

We had an account with telkom.It was cancelled on 15/3/18 ref nr ********** 26A Line was closed on 21/3/18.Another debit order went off beginning of April after lady said Telkom will actually owe us money.Another bill was sent beginning of May.Now my husband,Ian Freeman,ID ********** 062082 got an sms from VVM debt collectors that he should pay and amount of R900!!!!Our telkom account was cancelled.What the hec!!!!Please advise Helkom!And please rectify.If you blacklist my husband,I will sue you for slander and whatever inconvenience you have put us through!my husband’s number ********** 323