I suspect it is a call from an invention-snatching corporation.
I submitted an idea, then found out there would be thousands of dollars worth of fees to get the invention submitted to industry so I stopped short of signing the agreement. Now they email me incessantly and say they called me...the only un-accounted for number was this one, so that is why I suspect them.
I forward my unanswered calls to a disconnected number. Then I ignore calls like this one that I do not recognize.
That way, they tend to not call again (because they get the message that the number is disconnected)