Credit Card Recovery Call after 15 years

To my memory I had a credit card while I was in Chennai in 2000. I met with an accident and was bedridden in 2002. During this period i used to pay my monthly dues through a collection agent who comes home as I lost my job was unable to work. To the best of my knowledge I paid in full and remember him giving me a final receipt which I do not posses as I moved places and I am in a different city now. After close to 15 years today I had a call from the relevant bank asking me to pay up the dues while I was at work. So i told them I do not remember the payments if any and you may have to check your records. But after coming home I am so worried and recollect faintly that a full and final payment was collected by the agent and subsequently he produced a receipt as well (which is not with me anymore as life had so many changes and challenges in the last 15 years). But I am not aware whether the bank did receive the same or not. Was it a scam or is it a genuine call or how they find my current mobile no. Finally since it is a time barred recovery, can thy proceed with the collection procedure when I am not at fault. Please help.