Airtel — Excess rental charged

Airtel has been giving me a hard time and keeps sending me bills of[protected] every month. I am being charged for international roaming every month even though it was never activated by me.
The number was never available on international network but Airtel has charged me for the same.
On contacting customer care, they keep raising a ticket and say I will get a callback but that never happens and the ticket is closed.
Now they will even charge me for late payment as I am in no position to pay such a huge bill specifically when I never used the services.
Highly disappointed with Airtel. On trying to move out from Airtel, they are not even letting me do that stating illegitimate reasons.