The voice mail settings can be found under "Settings" and then "Phone settings".

In here you will be able to choose the agent that you need to setup the voice mail to by clicking on the microphone icon on the right side. 

At this point the browser will ask you to allow the use of the Microphone, just click on "allow" and then you can record your voice mail message. 

Once done you can click on "Stop" the recorded message will open in a new window so yo can hear it and once you decide to keep that one just click on "Save" on Ricochet.