Legal notice received by an IT Company and asked to pay FNF

Hello, I joined a company on the date 12/05/2014 and I resigned from it on 04/7/2014 immediately after 54 days around. But at the time of resignation I have informed my manager face to face that I am resigning and he asked me to send an email. He forwarded that email to concerned HR and other people. And I left the company without serving notice period and joined another for better career and opportunities. Few days after their F&F Team called me up and sent me an email asking to pay dues of around 18,328 INR. When I asked what this due, nobody explained me. Hence I never paid any due. However now after 3 years after I have received their legal notice to pay this due with 15% interest per year. Again I have sent them an email asking to explain me what is this due and how the due amt is calculated? But no one is responding me. I do not have their legal team's/F&F Team's phone number. Please advise what to do.