When using the mobile app, there isn't really a short cut. But, you can just click the red phone icon like shown below. If for some reason your phone has switched screens and you need to return to the call controls, just click this small keypad icon, and you'll then be returned to the call control screen.
Might be useful, but I was talking about your app for macbook. I often have a lot of pages and windows open and i was wondering if there was a shortcut to end a call, even if Ringcentral is not selected. Thanks!