Harassment and hanging up

These people really are terrible and unprofessional.

I had a Virgin Contract for my son 2 years ago. When it expired I told them I did not want to renew as we moved. They kept on phoning me and I told them I cancelled the contract, they kept on phoning up to 7 times a day, and every time a different person. My son wanted to take out another Contract with Virgin this year, but he could not, because I owed money. He paid it, they phoned me and thanked me for the payment and confirmed that the account was now settled. On Monday, 01 June 2020, they phoned me again to "thank" me for the payment and to tell me that I still owe R154,00 which I told them was impossible, as we settled the amount. Since then I keep getting phone calls from different people in the office, so, which means that they don't make notes or record the calls, cause the one does not know what the other is doing. Now I get at least 2 phone calls a day from them and as soon as I answer, they hang up.