I had to look it up, too, but it's legitimate. Banks in both Singapore
and the Philippines
(and I'm guessing others) offer virtual Amex cards that allow you to shop US stores that don't normally accept foreign-issued cards.
I assume you're using the GCash version, because that's the only virtual Amex I can find from the Philippines. I've never heard anyone ask for the phone number on the back of the card to verify ownership and, quite frankly, I don't know how they'd be able to verify a card based on what you tell them.
I suggest that a good first step would be to call customer service for GCash to see what they say.
[RIGHT][size=100]- Sapphire Preferred - Freedom - Ink - Platinum - Everyday Preferred -[/size]