Complain

I was handed over to them by cell c owing the amount of 7700,then we made arrangements and started paying 1000 every month from july so in January they call and tell me that im still owing 3700 instate of 1700 then i agreed with them that i will transfer 1500 and i did after transfer they debit 1500 again, so now last week they debit 1500 and still saying that im owing them 900,when i ask why they said they received another amount from cell c that owing 2000 so please help me im confused now