tee hee...I had to pull out the formula to see how it works
[250(80x+1) + y + y -250]/2
=(20000x + 250 + 2y - 250)/2
=(20000x + 2y)/2
=(20000x)/2 + 2y/2
=10000x + y
Basically you're just multiplying the first three digits of your phone number by 10000 and adding the last 4.

(who says there isn't a purpose for algebra!)