Panasonic KX-TA308 - Section 3 Telephone Features

2.9 System Data Clear
Installation 2-31
When the system does not operate properly after restarting, you can
clear the programming data stored in the system. The system will
restart with the default settings. First, try system program [999]. If
the system still does not operate properly, please follow the
procedure below.
Screws
3. Open the left side cover.
Left side cover
2. Remove the two screws.
1. Loosen the screw and open the right side cover.