DO NOT do this unless you have specifically been advised to.  This will render your phone inoperable until the reconfiguration is completed.  This is NOT a normal basic troubleshooting step.

This applies only to the older Grandstream models such as the GXP2100, GXP2124, or GXP2120.  For newer models such as the GXP2130, 2140, and 2160, please see this document:

If you are setting up a brand new phone out of the box, you can skip the factory reset section.  If there is any chance the phone was previously configured, you must do the reset.

Factory reset:

On the phone, press the center button in the five-button navigation keys to bring up the menu.  Scroll down to "Config" and press the center button to select it.  Scroll to "Factory reset" and select it.  Confirm by pressing the "OK" softkey.  The phone will clear its memory and reboot, which can take a few minutes.

Configure settings:

Once the phone reboots, note the IP address shown on the display just under the Grandstream logo.  Log into the phone on any web browser by typing the IP address into the URL bar.  The password and username are both "admin" (some phones may not ask for a username).  Click on the "Maintenance" option near the top right then "Upgrade and Provisioning."

Copy and paste this information into the "Config Server Path" field:

Make sure the "Config Upgrade via" setting is HTTP, not HTTPS.

No other settings need to be changed.  Click the "save and apply" button at the bottom of the page.  In a few seconds, the phone should reboot, acquire its settings, and reboot again.  This can take up to ten minutes to complete.