855-809-8123

Just received a call -- it was a legitimate call from our credit card company (Diners Club) regarding suspicious activity.  I called the company's regular Customer Service 800 number and they confirmed that the call from this other number was from them.

Just received a call from this number.  Said they were with Citibank.  Called Citibank # on my card.  The call was not from Citibank.  Probably not from Diners Club either.

Just received a call from this number--it was a legit call from Diner's Club Fraud Investigation Unit--they check on suspicious appearing charges. Called the Customer Service 800 number and confirmed this number was from Diner's Club

This call (855) 809-8123 was a legitimate call from Diner's Club Fraud Investigation Firm.*

Like others in this thread, I called Diner's Club directly using their 800 number and talked with a customer rep.
He looked into my latest charges after verifying that I was the account owner and found someone had tried to charge over $3,000 in Vietnam Air tickets. He stopped the card and issued a new one.

* The entry above concerning CitiBank might mean this firm is handling fraud for several companies,
  like Diner's Club, CitiBank, etc...