Found this on another site:

"This number is not actually UK (01992) 420745; it's US 00 1 992-420-745x

i.e. it's one of the rare US numbers that is having its caller ID sent over to the UK, but the last digit is being truncated as only 001992420745 is being read out by 1471. I know that means it's a digit short as I have to call the US at work and their numbers are all one digit longer than that.

Really annoyingly, I can't even manually add it to BT's Choose To Refuse that I have because you can't add foreign numbers to Choose To Refuse - even if you have the full number. I was going to add it and guess the last digit until I get it right and the calls stop, but can't."