Deactivate sound for iPhone calls on Mac OS Yosemite
There is the option in FaceTime preferences to choose a Ringtone.  Unfortunatly there is no option for "NONE".  As a workaround I found you can edit the file that sets this.  The file is com.apple.ToneLibrary.plist

The file is located in:

~/Library/Containers/com.apple.tonelibraryd/Data/Library/Preferences/com.apple.ToneLibrary.plist

If the file doesn't exist the below command will create it.  Running the below will not deactivate facetime calls only silence them.

sudo defaults write ~/Library/Containers/com.apple.tonelibraryd/Data/Library/Preferences/com.apple.ToneLibrary.plist ringtone "system:"


After you run this either logout and back in or run the below commands:

killall cfprefd
killall Finder


The part in the above command that is setting it to silent is the "system:", if you want to set it back to a sound ringtone you would do something like "system:crystal".  Where as crystal is the name of the ringtone.

Not sure if anything else is reliant upon this file so make a backup before hand if the file exist on your system.