A watch with Bluetooth calling has its own microphone and speaker, so you can answer and hold a conversation from your wrist while your phone stays in a bag, a pocket or on a desk across the room.

This is not the same as a watch that only shows you who is ringing. Notification-only watches let you see and reject a call; a Bluetooth calling watch actually carries the audio, routed from the phone over Bluetooth.

It works within Bluetooth range of the paired phone, typically around ten metres indoors. The phone still makes the call, so there is no separate SIM, number or airtime involved.