If you have already done this, then you can call your phone from a different phone and when you get to the voicemail, sign in from there and you can follow the steps to change it.
How to change voice mail password. to get your own password on the Iphone?
go to apple.com sihn in and go to iphone. click on apple sucks, you will then be transferd to microsoft to chat with bill gates about life as a small sudanees boy