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Apple Watch Series 3 (GPS + Cellular) models
Subject to cellular signal coverage, Apple Watch Series 3 (GPS + Cellular) can be used for cellular features when it is not connected to either the paired iPhone or a trusted known Wi-Fi network.
It has a built-in eSIM that copies information from the iPhone's SIM card. The built-in cellular features require a carrier plan for your iPhone that specifically includes support for Apple Watch Series 3 (GPS + Cellular), including the carrier activating the device on the plan. An activation fee and additional ongoing monthly charges may apply.
Contact your preferred supported carrier or visit their website for details of their service plans and charges for supporting Apple Watch Series 3 (GPS + Cellular):
Apple Watch - Carriers - Apple
Once it has been set up, Apple Watch Series 3 (GPS + Cellular) can be used for certain features entirely independently of the paired iPhone, potentially including when the iPhone is elsewhere and/or turned off:
How to use your Apple Watch without your iPhone nearby - Apple Support
Series 3 GPS-only models
Apple Watch Series 3 (GPS) models can be used for calls and texts on any cellular network that your iPhone is using, subject to connection requirements, potentially still including when it is away from the paired iPhone.
The full range of features is available when Apple Watch and iPhone are connected via Bluetooth and your iPhone has access to both cellular service and the Internet (via either cellular data or Wi-Fi).
You don't necessarily need to have your iPhone on or immediately around you (within arm's reach, for example) for the devices to establish and maintain their Bluetooth connection. Normal Bluetooth range is around 33 feet / 10 meters (this will vary in practice due to wireless interference).
Whenever your watch can't connect to the iPhone via Bluetooth, it will attempt, as a fallback, to connect to a known Wi-Fi network - this being a compatible 2.4GHz Wi-Fi network (some exceptions apply) that your iPhone has connected to before whilst your Apple Watch was connected to the iPhone via Bluetooth.
When instead connected to Wi-Fi, the watch also still offers a number of features, including receiving and sending messages via iMessage. If your carrier offers Wi-Fi Calling and it is supported on your plan and enabled on your iPhone, you can also use Apple Watch to make calls and (subject to carrier support) to send and receive SMS / text messages.
If your Apple Watch is unable to connect either via Bluetooth or Wi-Fi, it again still offers some - although fewer - features.
Features that are available in each case are detailed here:
How to use your Apple Watch without your iPhone nearby - Apple Support
For the best performance from your iPhone and Apple Watch (any model), including the most power-efficient communication between them, Apple recommends keeping both Bluetooth and Wi-Fi permanently enabled on your iPhone. More information:
More information:
Apple Watch - Compare - Apple
