Had same experience.  I had received a message to call that x5657 "Chase Fraud Prevention" number, but called the number (x5538) on the back of the card instead to confirm the fraud call and it indeed was a fraud investigation by Chase.  Apparently every time you use an access check they have to call and verify the check isn't fraudulent or they will not accept the check at Chase.  I guess I'd have to pay for the bounced check charges.  So I'd be really leery about using their access checks.  What if you're not available or don't have the card with you that day?  It should be stated on the access check offer that you have to be available by phone to verify the check over the next 72 hours or something.

This is the Number for Chase Credit Card Fraud Operations

does anyone know if this number is real? i really doubt that it is.

I also received a call from Chase Fraud Protection and was told to call back to 800-686-5657. I called the number on the back of my card, and that number is one of their legitimate Fraud Protection numbers. However, I'd still call the main number on the card.

I was told to call this number over suspected fraud on my Chase credit card.  When I called the legitimate number on the back of the credit card and was transferred immediately to Fraud Prevention.  This is the path I recommend.

If you are in doubt, call the number on the back of you Chase card to verify the legitimacy of this number.  I have done it twice.  They verified it both times and once forwarded me to that number.

I got a voice mail from 847-488-6000 to call 800-686-5657 concerning recent activity on my credit card. The card is in my name and they asked for my wife. I called my Chase credit card fraud department and the BP credit card company (only Chase card I have) - and they have no record of calling. They also advised me of not calling the number because it does not belong to them.

Tehy said they are from Chase and want to verify my credit card useage.
my phone is 302-284-2431  Larry

the number belongs to BankOne/Chase. It's their Credit Card Services Fraud Operations.