Sometimes I have to cycle the FaceTime setting to allow cellular calls. It is really buggy I think. I also read somewhere that all your devices have to be on the same WiFi channel (5ghz vs. 2.4 ghz) but haven't cared enough to test this as all my stuff is on 5.
Also you'll want bluetooth enabled for all your devices (don't have to be paired) if you want to use the remote personal hotspot feature. This is also really buggy for me....especially from Yosemite.