To change the default calling app on Mac, open FaceTime on Mac, select "FaceTime" > "Preferences" > "Settings".
Change the "Default" for calls to "FaceTime".
If that doesn't work, here's an app for it: http://www.rubicode.com/Software/RCDefaultApp/
You have to change the "tel:" handler.
You can find more explanation on this here: https://superuser.com/questions/548119/how-do-i-configure-custom-url-handlers-on-os-x