With the phone on the home screen, press and hold the OK button until the screen displays a warning message, "Reset to factory setting?"; press OK.

The phone will reset and reboot. 

Other Method

If your phone does not have an OK button or a directional pad (e.g. SIP-T57W Model) you must navigate through the menu items. 

1. Press Menu

2. Go to the Advanced Settings

3. It will prompt for the admin password (default is admin)

4. Navigate and press on Reset Config

5. Press Enter on the Reset to Factory Settings option