On the wall phones, you can adjust the volume on the handset (use the switch on the handset), but there is no manual switch to adjust the bell ringer.  

However, there are creative ways to lower volume by using something to obstruct the ringer bell e.g. tie tape around it to lower the ring or use something that touches the bell to reduce the ring sound.  

To completely disable to bell, you can simply open it (2 screws on each side of the jack), and cut off the red and black wires connecting to the bell to the circuit board.

Watch this video to see how to disable the ring bell.