Someone else paid our account...

We paid for a landline in January 2019, August 2019 they came to install our line. It went well for 30 days. Phoned them to confirm the amount we need to pay...They said that the account is in credit with R9000.... i told them I still need to pay because someone is going to bill us. She told me there is nothing we can do. They still cut the line. After phoning numerous times, I got no assistance and a demand for R3500!! I told them the story but they still insisted! Our lines are cut and we wanted to pay! We are so disgusted