(Not so) Unbelievable Math Problem

I got this in an email the other day.  I love these things. They used to drive me crazy. 
I know I know. "So that's what did it !!".  Anyway,  whenever I get one, I try to figure out just how they work.  They really aren't so Magic after all. Here's the actual text of the email.
  Here is a math trick so unbelievable that it will stump you. Personally I would like to know who came up with this and why that person is not running the country. I wonder what it means...
  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

And of course I wouldn't leave you without a handy dandy calculator, now would I ?

    

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Do you recognize the answer? Can you figure out why it's not magic?  I'll give you a hint.  It involves algebra and you have two variables.  You can do this!