Checking your sound settings in your Zoiper softphone is easy peasy! The picture below is pretty helpful too :)
1) Open up the softphone
2) Go to advanced
4) Make sure “input device” “output device” “input speaker device” and “output speaker device” are all set to the headset.
5) “Ringing device” can be set to the headset, unless you would like the ring tone to come through the speakers of the computer. If that is the case, be sure to make the speakers “default device” and the headset “default communication device” in the Windows sounds settings