Zoiper 3 does have a button to switch between headset device and a speaker device.
The button is located on the lower right corner on the main Zoiper screen, next to the "Missed" calls notification. It looks like a little circle.
To be able to use it, you need to configure your devices properly.
Please open Zoiper -> go to Settings -> Preferences -> Audio -> Device sub-tab -> as input/output device, please configure your headset. -> as Input Speaker and Output Speaker device you have to configure either the speaker device if it is an external one or the "built-in" if you wish to use the built-in speakers of the Mac.