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.