You can reset the Gigaset T300/T500 to factory default using the following procedure:

WEB-interface:

•     Download the empty backup here.
•     Open the WEB-GUI from the Gigaset T300/T500.
•     Go to "Configuration" - "Server" - "Backup"
•     Restore the backup from file, select the file dependent of your PBX version.
•     The PBX will be set to factory.

 Command line