How do I stop Vonage from resetting FaceTime settings?

I use a Vonage business VOIP line just to have an incoming landline number. I know, how quaint.

I always make calls using my iPhone and often like to do so using my Mac as I can make calls from a number on screen, hands free etc.

My problem is that the Vonage app resets my FaceTime settings, and makes Vonage my default calling app. I am now finding I cannot even reset the FaceTime default calling. How can I stop it doing this?