07725687199 Attempted Fraud HMRC

I just received a call with a recorded message asking me to phone HMRC urgently because I owe them some money. The number was a mobile

07725 687199 which is most suspicious of course. I called the number and a gruff sounding individual answered. I said I understood he said he was from HMRC but that I did not believe this and I invited him to prove it to me. His answer was in two short words the first of which begins with an F.