Note that one cannot simply click the green Talk button and adjust the volume. Sure, the bar count make it look like you're changing the ringer volume, but it doesn't seem to stay that way when you actually get an incoming call. The key detail on page 20 is that the volume must be adjusted on an actual incoming call.
Anyway, I experimented with both methods on page 20, adjusting during an incoming call and using Menu#160. I tried it on both the basestation phone#1 and phone#2 on the remote auxiliary charger. I wanted to be able to turn off the ringer at arbitrary times and turn it back on again. Well, I accomplished both, but one of the phones seems to have the icon for disabled ringing permanently displaying during an incoming call. It looks like a bell with a slash through it, and it shows on the lower left corner of the display. It rings, though. So I wonder what that icon is signifying, and why it only shows on one phone despite the fact that both phones ring. I cannot find that icon in the manual.