Learned Mr. Makkar has given valuable advice. Kindly follow it.
You do not owe any payment and you are under no obligation to approach your friend or company or listen to the calls of the company.
Your friend might have your number as a preferred number for free calling/ calling at subsidized rates at your number from his number.
Usually the telecom companies like Airtel play trick and make calls at frequently called numbers to pressurize the innocent and ignorant citizens to call the customer who has blocked their payment to contact the company and make the payment. This practice is illegal. In your case it is defamation of customer and you. You need not tell them that you shall contact your friend.
In majority of the cases the customer blocks the payment as neither the nodal and appellate offices of the company which are otherwise supposed to be unbiased resolve the complaint and do not penalize the company for its wrongdoings.
You may make a note of the date, time, phone number from which the call originated.
Next time if you receive any call you may record it , and speak the date, month, year, and ask the person to state his full name with sir name, name of his agency, address and then ask him to call airtel manager stationed at the agency, and repeat the same sequence and tell him that you are not owner of the number and do not owe money, and the employee of the agency under his control has threatened you stating.........(you may speak the threat given to you) and ask the manager to stop the calls at once or you shall be constrained to lodge a complaint with police and criminal case in court and go public by informing the media and may calla press conference. You should ask the manager to speak in whose name the phone number of the agency is registered and who is the proprietor/ director of the agency.
You can lodge a complaint with SHO, police commissioner and obtain complaint number on the copy of your complaint. Police can identify the number of the agency. Usually such numbers are not identifiable in the local directories. You can lodge a written complaint with local BM of the company's RO (Regional Office) in your city, and ask them to beg apology in writing and write the complete details of the calling person and their agency in their reply. You can lodge complaint at 198 which is complaint helpline number of the company and obtain complaint number and ask them to beg apology in writing. You should record all calls.
These companies indulge in such practices and continue since no citizen protest and inflict shame on the local staff in media and force them to make the details of the agency public.
If you are able to contact your friend you may advice him to put his grievance in writing to the company under acknowledgment and build record in his favor.