I am able to make and receive phone calls from my MacBook running Yosemite via my iPhone when I am only connected via WiFi to my home network (Airport Extreme).
When I connect additionally via my Thunderbolt Ethernet adapter to the same network it does not work anymore. Making and receiving phone calls always fails with the message "Call failed" after trying for some seconds to connect to the iPhone.
UPDATE: I can see the following error in the console:
17/10/14 14:15:23,082 discoveryd[56]: Basic Network GetExtendedFlags ioctl failed Device not configured
Best Answer
I did not realise that it works when I am connected only via Thunderbolt Ethernet - I though you have to connected to the same WiFi network.
You can only have one active network interface at a time.
Having multiple interfaces connected confuses the Mac and is in general a bad practice.
In short, pick one; either disconnect the TB Ethernet or shut off Wi-Fi.