In Skype for Business 2015 we have introduced the Call Monitor, which is a window that will be shown, if the main conversation window becomes hidden by other windows. From the Call Monitor you can mute or end the call and in a video call it will also show the video stream. This is a cool feature in itself, but often you need to get back to the conversation window for instance to check an IM or something similar. This is where it get's extra cool with the call monitor. If you double click in the area right to the photo and with the name and call timer, you will automatically get the conversation window shown. I've circled the area to double click in green in the picture below.