I was bored so I broke it down
math is fun ... if you let X be the first three digits of your phone number, and Y be the last four digits, you can rewrite those steps in the following equation:
(((80X + 1)*250) + 2Y) - 250) / 2
simplified to:
10,000X + Y = your phone number
so basically you're adding four zeros to the first three digits, and then adding your last four digits