Received calls from this number on May, 14, 15, 17, 19, 20, 2017. Caller ID shows as Private Name. Never leaves a message. I have a filter to block calls unfamiliar to me but they manage to get through anyway!

Caller ID said "American Express."  Left prerecorded message saying it wasn't a sales call but an "important message."  I don't have an American Express card.  I suspect this is a scam.

I've done some more checking.  The robocaller told me to call 1-888-819-2135.  I looked up this number on 800notes.com and there is a long history of scam calls pretending to be American Express related to this number.  I was once a fraud investigator, and my sister was a collections agent.  If legitimate bank or agency is calling you about your account, THEY WILL ASK FOR YOU BY NAME.  Do not give any information--including your name--if they don't.