This is not a fake number.
I tried to buy something online and my card got declined, I tried to get it 4 times thinking i must have entered something wrong but it was still declining. i then went online to speak to a natwest online agent, who told me to log into my natwest online banking, asked for the last 4 digits of my card and my name.
He then told me that the fraud team have detected some unusual activiity from my card and have put a temporary stop on my account until they can discuss with me my recent transactions. They gave me the number for me to give them a call.
I gave the number a call. The lady that picked up the phone asked for my full card number, my date of birth, the first line of my address and my branch I was with. Once i verified my details, she told me the most recent transactions (the last 3) and wanted to check that these were all me. All of the prices and places that I had bought things from were correct so I confrimed that they were all me. She said that my recent transactions followed a fraud thing so that my account had been flagged up. Once she confirmed that all the transactions were me she said that she will now take the block off it.
I then tried to purchase my item online again and the transaction went through!