Sony IT-M804 - Manual (Page 75)

75 US Additional Information
Additional Information
Note
When you reset the phone by pressing RESET button, the whole contents Phone
Directory, area codes, Caller ID data, messages, the dialing mode and Do Not
Disturb settings may be erased.
To return the phone to the factory setting
When you lost the security code or need to erase the home area code,
etc., reset the phone by following the procedure below.
Note
If you return the phone to the factory setting, the whole contents of one-touch
dial, Phone Directory, Caller ID data and all settings will be erased.
To return the phone to the factory setting, unplug the AC power
adaptor, and then plug it while pressing (ERASE/CLEAR). If the battery
is still inserted in the phone, take it out first.