What amount of Bandwidth to allocate for 3 to 4 concurrent calls and how to setup in SonicWall

We have a small office of about 20 VOIP ringcentral phones.  I think that at max, 3 concurrent phone calls happen at a given time.  Perhaps 4 on the busiest moment.  What would be the appropriate amount of bandwidth to allocate for Qos on my sonicwall Bandwidth Management screen.  I have a hard time understanding how this screen is explained so if someone is able to explain what Realtime is and Medium and Guranteed vs. Maximum/Burst that would help.  Thanks
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