I have received legal threat from Internet service provider on billing

Hi I'm customer of tata photon from 2011, on dec 2016 I called them to cancel , but they convinced me to opt for safe custody ( 6 months ideal, after that they will call me and confirm before reactivation). After 6 months I received follwing mail, "We wish to inform you that the safe custody feature on your number 924xxxxxx is getting over on 24-JUN-17.We would like to have you back on our network and would request you to call on our customer care to activate your connection." I didnot reply to this email or called their customer care as I was not interested, But after 20 days they sent me another mail with bill(rs.300), at that time only I was aware that they have re-activated the connection without my permission. Again I called their customer care asked to cancel the connection, I paid the 300 bill. Since the cancelling took another 10-15days again they sent me a bill for Rs.700. I did not pay the last bill, today they called and threatning me that they take legal action. Pls help