use of abusive language by the company staff

Location/place: jodhpur

Name of company/service: airtel

the company staff is ringing repeatedly and using abusive language to us who are one of its oldest customers since last eighteen years.Having failed to deposit the bill on time once has the company entitled it staff to use these tactics? I WISH TO KNOW if this was under the company instructions or the staff was using their own power and if the company denies its involvement then where can I complain about them. I hope for a reply