I just recently upgraded OSX to Yosemite. Right after the installation, there were an incoming call to my iPhone, and the incoming call also appear on my freshly installed Yosemite, the Facetime pop up and asked whether I want to answer the call. I did clicked on answer call, because I thought it was a Facetime call from my friend, but it was actually a voice call.
The feature is great, but without a proper headphone with mic, your conversion may be heard by people around you. I preferred to have it disabled, it took me quite some time to find out how to disable and stop phone calling to Facetime. Here are the steps to disable phone call in OSX Yosemite
1. Fire up your Facetime application.
2. Go to Facetime > Preferences OR Apple + “,”
3. Uncheck “iPhone Cellular Calls” from the preferences.
Now all the calling happening on your iPhone will not ring your Facetime in OSX Yosemite. Below is the video showing how to stop phone calling in OSX Yosemite.