OK - here is the process to set the ringer volume - on that handset.
1) Pick up the handset while the system is not in use (not on a call).
2) Press MENU # ***** 6 0
3) Use the UP or DOWN arrows to change the ringer volume to the desired setting.
4) Press SAVE.
5) Press OFF.
As an example, I have a link to a video that shows this same process. NOTE: The handset in your system may NOT match exactly (different number of buttons, etc.), but the process is the same.
Click Here --> https://www.youtube.com/watch?v=W0oYPkVGp2M
Please let me know if that works for you or if you have any more questions.
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Thanks, ***** *****