The 3900 series digital phones and the 1100 series voip phones don't allow you to turn off the ringer all the way with volume control on the phone set. (Funny, we could do that 30 years ago with analog phones.)
If you never want the line to ring, re-program the line as a non-ringing appearance (SCN key) as suggested above.
If you want to toggle ringing on and off like a do-not-disturb feature at certain times, use the Forward key to forward calls to voicemail and the phone will not ring as long as it is forwarded. Cancel Forward to receive calls again.
I share your frustration. Hope this helps!