How do I inform Halifax my landlord is BTL on residential?

I received a letter from the Halifax this week to my landlord, I opened it by accident and found out my landlord is on a residential mortgage.

we moved in right after he purchased it so he never lived here,

does anyone know how I can inform the halifax that he is renting it out? I rang the number on the letter but the lady at the halifax was a bit thick and didn't understand what I was getting at.

Is there a fraud number for landlords anywhere?