Fun with numbers

Received in an e-mail:

UNBELIEVABLE MATH PROBLEM

Here is a math trick so unbelievable that it will stump you.
Personally I would like to know who came up with this. It is pretty
amazing
1. Grab a calculator. (you won't be able to do this one in your
head)
2. Key in the first three digits of your phone number (NOT the
area code)
3. Multiply by 80
4. Add 1
5. Multiply by 250
6. Add the last 4 digits of your phone number
7. Add the last 4 digits of your phone number again.
8. Subtract 250
9. Divide number by 2
Do you recognize the answer?