Now i have checked with my exchange and got the clear picture.I had taken my "Home 500C" broadband connection in June along with my new land line connection.In the first bill they had taken only broadband+model security deposits of Rs 1000/- but from July they were had to take security deposit for land line (Rs 1000/-) in 3 months. But they did not took it it July. And starting from Aug to Oct, every month they are taking Rs 333/- for the deposit.So, this was my last deposit for security. From Nov I'll ONLY pay the usage.I really thankful to my exchange head mam for her help to explain me in detail.Thank you.