Phone number puzzle from 2000

[Bill Stokes]

I read about this years ago, and knocked up a perl script to do it on line 16 years ago - resurrected this year.

I can do it my head, and goes down well in pubs etc. (if people can subtract correctly, which alas today seems an almost impossible thing for people to do without a calculator!).

https://linicks.net/ptrick.html

Nick