I just got the same - a call with no message left.  I've been inundated with calls from a notorious scam charity this week so I suspect it's them.  They will call with an ID of "St Jude" or "Womens Cancer" or some similar worthy-sounding cause and when my call blocker rejects it, they will call again immediately with a caller ID of Portland or Bangor with a local number to get around the blocker.  Scumbags!  I have no idea why they keep calling me - I've never donated to any of their fraudulent charities or even answered one of their calls - but they are relentless and won't stop calling.