All Gigaset Hansets are reset to factory default by the following:
• Switch off handset
• Hold down the keys 1, 4, 7 and switch the handset on
• “Service” will be displayed
• Enter 4 6 8 5 4 6 3
• The handset switches off
http://me.go-unified.com/access-hidden-menu-of-siemens-gigaset-cordless-devices/
