Hi
Apple Watch does not have cellular capabilities of its own, so cannot be used for cellular calls or text messages via cellular service unless it is connected to the paired iPhone.
Apple Watch offers the widest range of features when it is connected to the paired iPhone via Bluetooth or Wi-Fi. For the best performance, keep both Bluetooth and Wi-Fi permanently enabled on your iPhone.
Apple Watch still offers a number of features when the paired iPhone is either unavailable (eg turned off) or out of connection range:
- Use the watch to tell the time, along with alarms, timers and the stopwatch.
- Keep track of your activity (Stand, Move, Exercise) with the Activity app.
- Track workouts with the Workout app.
- See your heart rate.
- View photos from the synced photo album.
- Use Apple Pay to make purchases.
- Play music from a playlist that has been synced to Apple Watch (Bluetooth headphones are required).
If the watch is not connected to your iPhone via Bluetooth, but is within range of a compatible 2.4GHz Wi-Fi network (some exceptions apply) that your iPhone has connected to before whilst your watch was connected to the iPhone via Bluetooth, then the watch can automatically connect to that Wi-Fi network itself and you can also still:
- Send and receive messages using iMessage.
- Send and receive Digital Touch messages.
- Use Siri.
- Make and receive phone calls and send and receive SMS text messages (if you have Wi-Fi calling enabled and these features are supported by your carrier).
- Check the weather.
- Track stocks.
More information:
How to use your Apple Watch without your iPhone nearby - Apple Support
About Bluetooth and Wi-Fi on Apple Watch - Apple Support