Apple introduced Continuity as a way to bind together various features across the two kinds of systems. It’s great when it works, and it’s clearly gotten better. AirDrop only fails on me sometimes instead of mostly, for example. Continuity includes Instant Hotspot, Universal Clipboard, and other features.
But Macworld reader Tom is having trouble with the Cellular Calls part of Continuity, which effectively extends an iPhone voice calling service to work with any associated Mac or iOS device. He has both Wi-Fi and ethernet active on his Mac laptop when he’s using it at home, and phone calls ring on his Mac through Continuity. However, if he has both ethernet and Wi-Fi active, he can never answer the call: FaceTime reports the call has failed. Disable one networking method, and it works.