Phone Number Trick

Here's a math trick so unbelievable that it will stump you. Grab a calculator 'cos you won't be able to do this one in your head.

1. Key in the first three digits of your phone number (NOT the Area code...)
2. Multiply by 80
3. Add 1
4. Multiply by 250
5. Add to this the last 4 digits of your phone number
6. Add to this the last 4 digits of your phone number again
7. Subtract 250
8. Divide number by 2

Do you recognize the answer ???

I tried it with a few phone numbers and it worked every time!

The numbers are random so could anyone care to enlighten me as to how it's done?