Does he leave belligerent messages?
If not, just don't pick up.
If so, get your cell company police department involved, back trace, and fuck the guy over of his job.
"He" = an outsourced India call center. I've received several calls from them, from different "agents". It's not the same person, but is the same company. Unfortunately by showing as "Unknown", I risk missing other calls that may be important.
He doesn't leave belligerent messages, but rather
is belligerent on the phone (bill collector, single minded tunnel vision that will simply accept nothing other than you saying you're the party they're looking for and you're willing to pay, even if you're not [either]).
Getting the police department involved will likely net nothing, given that
this is a call center in India.