Airtel Cellular — anonymous and awful messages

Hi, i'm complaining about an airtel number, [protected] for my friend. She getting anonymous calls and messages from this above mentioned number to her tata docomo number. She really annoyed. She requested them to know about whats wrong with them. But they refused and they sending awful statements such as I will kill you and etc. So please help her in this. She would be very grateful to you if you help her. Thanks. Regards, rajesh