In order to factory reset a Polycom handset via the phone interface, go through the following steps below:

• Press the Menu or Home button
• Select Settings > Advanced. The default admin password is 456
• Select Administration Settings
• Select Reset to Default
• Select Reset to Factory 
• Select Yes to confirm

Wait for the phone to reboot. Once done, the device will be reset back to factory settings.

If you don't know the phone's password, then you can use the MAC address, by following the steps below:

• Power off the phone by unplugging the power or network
• Plug the phone back in
• Wait until you see Cancel, then press Cancel button
• Quickly press and hold down the following keys: