Unsolicited "Research Firm" again as my Oct. 9 entry here. This time they left computerized voicemail which started with "that was an invalid response, please press 1 if you are male, 2 if you are female". This continued for five iterations until timing out.
Honestly, my best guess is a debt collector trying to see if the number is valid and gain personal information because the call is usually surrounded in quick succession with an unknown caller ID - too frequent to be coincidence.