Improper billing and harassment by the airtel recovery

Hi, I was using airtel post paid internet data connection no 1426 (mumbai) for 7-8 months. My credit limit is rs.1100 my usage was never more than 500rs.(As per data plan) due to very poor network issue, I registered complaints several time but never got a response.. suddenly next month I got a bill of rs. 1656.40..!! I called up the customer care many times to get the issue sorted & stop my connection but they never came back to me with a solution. Next month, I got a bill of exactly same amount rs.1656.40, after that I stopped the connection. After that obviously case was escalated to recovery dept. After lot of discussion with recovery agents (mr mehmood - 9967865340)..he offered me to settle the account by paying 900 rs. Which I DID.. considering a correct amount to be paid. Now after 2 months, I started getting calls from delhi (vikram rana- 158) asking for outstanding amount of rs. 2600..! I'M not getting.. why should I pay now...!!! rescenty I got a legal notice and they are asking me to reach bandra court on 15th sept. (Tomorrow), I dont know should I go there..how can I put up my case without any lawyer.. also I wont affort to hire a advocate for such a matter... I want to registered a complaint for harassing & using abusive language too.. it is a very sad part.. request you to please help me..& suggest me a way forward.