Algebra

A=first 3 digits B=last 4 digits

((80A + 1) X 250 + 2B - 250)/2 is equation representing your instructions - the  reduce it to it's simplest form

(20,000A + 250 + 2B -250)/2

10,000A + B which puts the first 3 digits 5 places to the left of the decimal point, and B right after it, giving you your phone number back.