I reviewed VoxOx about 5 years ago. It was awful. It is still awful. The latest Mac version will actually make and receive calls on OS 10.10.3, but not without problems. It is not possible to access voicemail -- the vm window is blank. It is not possible to set some of the important preferences as that window is also blank in places.
As for the quality of support, there is none. I recently posted a question to the VoxOx online support forum about the vm problem, and it was deleted. (I didn't include any editorial comment, just a question.)
Astonishingly, the VoxOx CEO was a finalist for the San Diego Business Journal's Most Admired CEO award, handed out in 2015. That tells you all you need to know about that Journal's credibility. The CEO is apparently even invited to gives speeches at tech conferences. I bet those speeches are not about how great VoxOx is on a Mac, or the company's excellent support.
On the plus side, it seems very unlikely that more than a few masochists actually have VoxOx on their Macs.