Zoom Phone receiving calls

We are new users just trying to tweak our system to work the way we need it. Our current issue is when one of our people are on the line, it won't register an incoming phone call to that line, nor will it put the missed call in that line's history. There are times during the day when only one person is tending the phones (staggered shifts early and late in day). In this circumstance they don't know what they have missed a call, unless a voice message is left by the caller.