If you tried to change your phone's permissions but were unable to do so, it is likely that your phone still has a security configuration left over from a previous user.
If your BlackBerry was previously activated on a BES corporate network and the restrictions were not deactivated, the only way to clear the permissions is to reset your phone to factory defaults.
You will need to use a computer to complete this process. If you do not have access to a computer, you may want to go to one of your mobile provider's shops for assistance.
Warning: This process will remove all data from your phone, including settings, contacts and media. Please back up your phone before continuing.
You can learn how to reset your phone to factory defaults in this BlackBerry Knowledge Base article.