Hitachi TD61-2472 - Manual (Page 58)

1.11.12 KeyLock/KeyUnlock
When the keylock is set, button operations are disabled.
1 Press and hold down the Lock key on the side of the phone.
Enter the password.
Note! The default value for the password is "0000". You can change the
password freely. (See the security section.)
2 The keylock comes On.
Hitachi Cable 54