Though you raise a cancellation request with Airtel, they never take it forward easily. After constant follow-up they will register the request, all the while making you pay for the un-used services. As if this was not enough, the actual date of cancellation of services is always taken 7 days after the request is officially registered.
They also take up these cheap tactics of "threatening calls" from the collections team even if the collection amount is only couple of hundreds. It is so sad that in India there is no law to curtail such hooligan acts.
My suggestions to everyone: have a well documented evidence either through email or a written communication which you could get acknowledged at one of their customer service centers and never pay anything above than what is legally due.