To adjust the notification volume of an incoming call, you must right-click the Ring Down connection. The Ring Volume slider will be displayed (Figure 1):
Figure 1 Ring Volume
Adjust the Ring Volume slide as required. This will change the volume for all rings on that connection moving forward.
You can mute the volume completely by selecting the Mute icon, or bringing the slider all the way to the left (Figure 2 and Figure 3):
Figure 2 Mute
Figure 3 Muted Ring Volume slider