iPhone cellular call relay security
When a user’s Mac, iPad, or HomePod is on the same Wi-Fi network as their iPhone, it can make and receive phone calls using the cellular connection on iPhone. Configuration requires the devices to be signed in to both iCloud and FaceTime using the same Apple Account.
When an incoming call arrives, all configured devices are notified using the Apple Push Notification service (APNs), with each notification using the same end-to-end encryption as iMessage. Devices that are on the same network present the incoming call notification user interface. When the user answers the call, the audio is seamlessly transmitted from the user’s iPhone using a secure peer-to-peer connection between the two devices.
When a call is answered on one device, ringing of nearby iCloud-paired devices is terminated by briefly advertising using Bluetooth Low Energy (BLE). The advertising bytes are encrypted using the same method as Handoff advertisements.
Outgoing calls are also relayed to iPhone using APNs, and audio is similarly transmitted over the secure peer-to-peer link between devices. Users can disable phone call relay on a device by turning off iPhone Cellular Calls in FaceTime settings.