You will need paper and pen or just calculator to perform this trick.
1. Put first three digits of your phone number in calculator
2. Multiply that number with 80
3. Add 1
4. Multiply with 250
5. Add last 4 digits of your number to the number you got so far
6. Again, add last 4 digits
7. Subtrack 250
8. Divide it by 2
9. You got your number
* It is said that this only works with seven-digit numbers.