Start a call by dialling from the Dial Screen.
Press O to get the Android Main Menu screen.
Select Firefox and look at some stuff
Close Firefox, back to Android Main Menu, select Zoiper.
The Dial Screen now offers you the standard 'No call in progress' style screen with 'xxx is ready' and a dial pad. There is nothing to hang up the call you have in progress.
The only way to do it is to start another call that is bound to fail, hang that up, then you get the opportunity to hangup the original call. A very clunky work around.
If you try to just Exit Zoiper it says "Ah, you can't do that, you have a call in progress". Why did you think I wanted to exit Zoiper? Apps that know better than you about what you want to do are very frustrating.