Use reset to factory default (select it from phone's menu) then configure it from scratch.
If phone use remote provisioning, then you can use provisioning to change password as well.
Phone may be configured not to allow unauthorized access - and you are unauthorized person (as you know no correct password).
If reset to factory default is protected by admin password (which you don't know) and phone use no remote provisioning (or you can't change the provisioning file) you are definitely out of luck.
