it only works if you have a 7 digit number I think

here, I've got a simple one but trust me - it's the same principle

1. grab a calculator (or even do it in your head)
2. key in they number
3. add 1
4. substract 1

see what I mean?

you have your number since the beginning anyway.
would be cool if you start with your phone number and end up with your friend's phone number or something like that