Airtel — recharge needed even for incoming calls

I had been using the number for many years, and it was an life time plan. But in the last few months, my balance gets deducted stating incoming and outgoing facility has expired. I hardly have any outgoing calls to make and therefore I do not understand the need for any such balance deductions. I make a call or two every 1/2 months, and I think that is sufficient to maintain a mobile number active, then why is the balance getting deducted.