When I can't identify the area code and/or number when it appears on the call display, I don't answer.  If it is important they'll leave a message and I return the call immediately. The nuisance/telemarketing  calls all hang up before my call answer kicks in.  I traced this one's source  to Morley, Michigan (I live in Canada)  Michigan is a hot bed of telemarketing calls.

do not know caller