Check out this calculator trick I found:
1) Put the first 3 digits of your phone number in (if you're British put the last number of your area code and then the first two of your main number)
2) Times that by 80
3) Add 1 to that answer
4) Now, times that number by 250
5) Add on the last four digits of your number
6) Repeat Step 5
7) Minus 250 from your answer
8) Divide your new answer by two
Is that your phone number? (If you're British, ignore the first number!)