Continuity limitations
Calls are received as a normal call on your phone and then it uses wifi to tell other devices on the same network that a phone call is incoming. If accepted on other devices, the phone is used as a proxy. Voice data is transferred from the device to the iPhone over wifi and then to the cellular network and vice versa. If your phone dies the call will drop. If your phone goes out of wifi, I think the call will be transferred to your iPhone.

This is different from Wi-Fi calling which doesn't require your iPhone on the same network.