Disabling the incoming call feature technically puts the Zoiper application in shutdown mode as there is nothing for it to do while in the background.
The reports of ticking noise of iPhone plague also the apple forums and maybe there are other applications that generate it. In my case, however, going through the list of applications and switching off Zoiper fixes it. Obviously, disabling background function would do the same, but simply means that you acknoledge the aplication as being buggy and need to do something about it.
Please identify weather you have a leftover in the code that triggers a sound function or a re-focus on the camera.
Seeing other comments, it is reproducible on my phone and my wife's phone and does appear after various calls.