Someone called me from this number and left a message about fraud early warning. I called citibank directly and they have no record of them calling me. I would not trust this number. Just don't give them any info and call the number on the back of the card.
For me, this was definitely Citibank's fraud protection line. I had earlier made a couple of overseas flight reservations, so because the charges were made via online AND were from out of the country, the fraud protection department was calling to verify that I had made those charges. They knew the exact amounts, etc. It was legit, and I appreciate the extra protection.