Simple maths or a trick?

Here is a math trick so unbelievable that it will stump you. Personally
I would like to know who came up with this.

1. grab a calculator (you won't be able to do this one in your head)

2. key in the first 3 digits of your telephone number (landline not your
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???

NOspam...

No!
why?

Come on all you geniuses out there, please explain this one to me...