Sorry, but that is not what is happening. There is no "ring volume" setting in an OBi device.
When a call comes in, the OBi sends a ring signal to the phone. It's up to the phone itself to do the ringing. If the phone has a setting to control the ring volume, (e.g high, low, off), you can use that switch.
If you have a very old telephone with an electromagnetic bell ringer inside, then the ring signal from the OBi is powering the bell. If you have a typical, newer phone, then there is a circuit inside the phone that powers an internal speaker or beeper to make a ring sound.
What kind of phone is it? You can easily apply the low-tech solution: put something over the speaker/beeper, like a cotton ball, small wad of tissue, or tape.