The Mac isn't receiving the call, the phone is. The Phone is just handing off the UI to the Mac across Bluetooth and Wifi. If the phones worked this way, if his wife walked out of the house with his phone, and let's be clear at that point there might not even be a way for her to tell she's doing it, it would drop off the local network and incoming and outgoing calls would become impossible.
Maybe there are limitations like needing to be on the same WiFi network or some such.