Sometimes once a call gets started on my tablet, I can't seem to cutit off (hang up the call)... because the call screen with the red 'endcall' button is nowhere to be found? How do I find the call screen, orotherwise hang up the call?
On android if you have moved away from your call screen, to return, simply open the notification center of your device and tap on the currently activate call. This will return you and you will be able to hang the call.
For me no notification shows up at all during call; I think this is some kind of bug.
If you want to quickly end the call, you have to manually kill Zoiper. Long press the application icon on your launcher > Information > Force close the app.