Airtel — unwarranted sim service expiry

Suddenly received a msg from airtel that my outgoing service will expire soon if i didn't recharge, my account already has rs100 talktime o[censored]nlimited validity, i did recharge for it on 25th september and recharge before that was done in january. Explain this too me airtel, how can validity expire on a lifetime valid sim. Now all of a sudden, the service terms have changed! Stop fooling your customers for forceful recharges