This applies to all current Grandstream phone models such as the GXP2130, 2140, and 2160.  It also applies to some older models, but the menus may look different.

To lock the ringer volume so it cannot be changed on the phone itself, first log into the phone's web interface.  To do this, get the IP address of the phone by pressing the UP key on the directional pad.  (Older phones with a non-color display usually show their IP address right on the main screen and this step doesn't apply.)

The display will show the IP address of the phone, which will look similar to this:  Type the address shown into your web browser, which will then display the phone's login screen.  The default login is "admin" for both the username and password.  If you are unable to connect to the phone, please see the troubleshooting section at the end of this article.

Now navigate through the menus as shown below to set the desired ringer volume (step 4) and then lock it (step 5).  Use the save and apply button, and now the volume will be locked to whatever you set in step 4.