how can the tax office NOT know your NI number, sounds lke you have been scammed

More to the point, how would they know your phone number but not your NI? Smells like a big fat scam to me!

The DWP don't do anything by phone. If they wanted information from you they'd write. You'd do well to contact them & tell them what's happened. If it was genuine they'll be able to tell you. If not they'll be very interested to hear about it.

Not sure how they do it in the UK but in the states they will never call you they only send out letters only with a number and what it is about...

Here if they call you it is a scam and I would call and tell them of the phone call that you got from them and what info you released..

Last time I called them I had to confirm my NI number, which I did and they insisted it was wrong. I insisted I was right and they said I didn't sound like a 69yo man! I finally guessed the right one and learned it off by heart as soon as I set down the phone.

I very much doubt they would have phoned you though. They would have contacted your employer to confirm which code you should be on and then would send you a letter in the post to confirm any changes they made. They wouldn't want any info off you as they would ultimately have to confirm the details of employment with your employer.

(I should point out that the above account details one time I was wrong. There are no further records to suggest this has ever happened again.)

Nobody ever asked for my N.I over the phone.

probably been done over already. best thing to do when you get asked for the number over the phone is mix up the numbers and see if they tell u its wrong. normally will get a letter to tell u its the wrong tax code, never a phone call!