In a conference call using Skype for Business 2016/2015 desktop client, an end-user is only allowed to toggle between Speaker and Gallery view using the left most icon on the top right hand corner of the meeting window. Until someone shares something or makes a PowerPoint presentation, you may have to stare at the (typically low-resolution) overblown photograph of the active speaker or a row of meeting participants’ pictures.
To go back to classic “telephone” audio voice calls with no visual cues of the person speaking whatsoever, you can hit Ctrl+Shift+P on the keyboard to move into Compact View for the Sfb desktop client. This functions apparently only if the Participants window is active otherwise the Gallery view will be shown.