Please use a calculator. This cannot be done in your head.

1. Input the first 3 digits of your phone number (not the area code).
2. Multiply by 80.
3. Add 1.
4. Multiply by 250.
5. Add the last 4 digits of your phone number.
6. Add the last 4 digits of your phone number again.
7. Subtract 250.
8. Divide by 2.
9. Output result.

Do you recognize the output?

(Source: CuriousMath.com at "Phone Number Trick")