On my iPhone I have the Cellular data turned on and under the Cellular data tab under Settings I also have 'Allow calls on other devices' turned on. Then under 'Allow calls on' i have my iPad and my desktop computer turned on. When I receive a phone call all 3 devices ring and Facetime opens on my desktop. I get a panel allowing me to accept or cancel the phone call whether it is a Facetime call or not. If I click on accept, the phone call is answered, and I can talk to the caller.
Under Facetime preferences/Settings, I have all the boxes checked under the 'You can be reached for Facetime at:' settings. I don't know if this helps you but this works for me.