called at least once before, no message -- didn't return call but added to reject list.  I don't recognize the number.  area code is in Georgia.  

No message but when I return call they say they are American Express.  I'm not sure if it is a scam or a bill collector, but I don't think this is a valid number for AMEX.  

If unsure, always look the number up. On your card, statement on on their (dot) com web site. Caller ID can be faked & you cannot guarantee the caller is who they say they are. I would consider it unwise to call a number given to you in an email, text, or voice mail. Especially if it involves personal, or financial information.

Calls probably once a month to my cell. I have an Amex charge off from 6 years ago. Probably them as I understand Amex doesn't sell your debt to third parties.

Calls made to my cell phone, about once a month in the past 3 months.