Consumer Redressal

They had taken advance payment twice. that is one extra payment already had been paid since I went for a change of scheme.At the time of installation they had taken advance payment. For change of scheme also they took advance payment asking me to clear the bill.I cleared the payments as asked by them since I needed the service.In fact they should not have billed me if they were asking for the advance. Here they took both. At closure now they are demanding payment once again which I have denied. Threatening calls are coming from Delhi. Kindly advise me in the matter and help.