To hack a voice mail password on a cell phone can some times easy. Most cell phones come with a universal password. These passwords are 0000, 1234, 9999, the last four digits of the cell number or the last 4 digits of the persons social security number. If you have tried all of these options then the owner of the cell phone has changed the password to their personal password. If this is the case
. the only option you would have at this point is to guess. The cell phone company can reset these passwords but if you are not the owner of the account you will not be able to have this done.