Hello Steven,

with the volume rockers it should be possible to set ringing to a low volume and finally to only vibration.
To turn off ringing immediately on an incoming call please try pressing the on/off button of the smartphone while it is ringing.
Within myPBX Android you may choose to switch on "Do not disturb" in the config screen. It will signal "busy" to the caller immediately.

The slider is not functional indeed. We rely on the smartphone buttons to control ringing.

Best regards ... Thomas