Mtn very bad

Very bad services. When i cancelled my contract, they sent me a invoice for upgrading. Keep on calling them but no one knows what his/her job is and no one can assist with the problem. Keep on mailing them, no reply. Then they hand you over to the attorneys for collections. I cannot get financing for a vehicle because of this issue. Keep on telling them that they are busy drowning my bussiness, but they just dont care. Dont know what to do