well you'll need a dsc-11 service cable, aka: t28 Unlock cable first. and if your looking for a free way to do it you'll be searching forever if your willing to spend a little money to get your phone unlocked then you can use a remote server solution like: Total-Multi Server or Davinci. I personally use and sell Total-Multi Server credits and feel it's a better software.
if you want more info about that click on one of my signature links.
good luck man! and make sure to post your results when you get your phone unlocked.
however, there has to be a way to unlock Z800i without paying to the phone shops. Most of the unlocking companies and phone shops provide unlocking via codes. If you type