888-200-5598

They said they were from Security Service (my bank) and were warning me of potential charges to my account

These are low-lifes warning about unverified purchases on your bank account
Dont fall for this
Just call your bank...Dont answer their phone calls..
Better yet, answer but dont talk. They waste my time, I am going to waste their time

You're a moron.  It's MasterCard's transaction services calling to verify fraudulent appearing charges on your account.  Once they've gone over the questionable charges, they transfer you straight to Security Service so a new card can be issued if it's determined that the charges were indeed fraudulent.  You should be thankful that they're so watchful of potentially fraudulent charges.  They've caught thousands of dollars worth of charges on my account.

If they are for real,  then it won't hurt a thing to call the bank first and verify,  get a toll free number in which to call them,  then initiate a call  to follow up on their phone call.   I always do this when I get a call with reference to a charge card purchase.  It offends no one and so far,  the two or three times I have been called,   it proved to be a legitimate call verifying a purchase on my card.  
I don't think calling anyone a moron clarifies any issue.  Just poor manners.

This is a REAL card services number. They called me and I almost blew them off but when I called the number back and checked my bank statement they were questioning a $559.96 Walgreens purchase made in another state... I didn't purchase anything at a Walgreens in any state and it saved me a lot of hassle to take care of it right away and get my card replaced before they could have more fun. As soon as the purchase popped up it red flagged and they called us that fast!!!

The best part was it did not cost me anything so this is the real card services for Mastercard. I left call type as unknown since there is not an option for legit calls.

This is a legitimate number. I had 3 transactions that were legitimate but rare, one of them to a local charity that I donate to once a year. If you get a call from them, definitely call them back.

After looking up the number and reading these post I called them back.....they were verifying charges to purchases I had just made a few minutes before their call. They even asked about purchases from last night and gave me the business names and amounts.