MTS / Sistema Shyam TeleServices [SSTL] — harassing and threatening from unknown mobile number

Hii,
I've been getting calls from mobile no. [protected] from, my friend even gave a call to this no. And asked not to do this again, still i'm getting call from this number threatening and harassing me it is very irritating and harassing me mentally.

Please look into this matter as early as possible. Since i have zero balance on my mobile i called him through landline to warn him and scold him, but he ignored and started laughing taking it lightly, please take necessary action against such people.

Regards
Archana