Amazing Maths!

Key-in the first 4 digit of your handphone number into the calculator
Multiply by 80
Plus 1

Multiply by 250
Plus last four digit of phone number
Plus last four digit of phone number again
Minus 250
Divide by 2

Is that your mobile number?

It works for mobile numbers in Singapore.
At least for mine and my wife's.
Not sure about mobile numbers in other countries.