Airtel — mobile number validity

Dears,
I have a serious issue/ complaint about airtel. Last month in my airtel prepaid number, it shows the validity upto 2021. Now today i have received a message which is telling that i have to recharge for my mobile number validity by 18th of december this month after that it will not work. If this type of services they will give every time then i don't have any option except to quit from airtel services. Every time i have recharged my number when it goes down & even i have not used it for calling as it is in international roaming. But they deducted from my account every time even i have not taken any services. Kindly solve the issue as soon as possible.