1. Turn on your phone with a SIM card that's not from the original network, IE not locked to
2. The phone should show the message 'SIM VERROU'
3. You can now enter the Unlock Code provided.
4. The phone should now be unlocked.
If you see the message 'CODE POSTE', it's because the phone has a personal Security Code enabled (IE a PIN)
By default it's usually 0 0 0 0 or 1 2 3 4, but alas this could be anything as it's user created.