Apple Watch texting and calling
Do I need to buy a separate data plan for texting and calling to work on an Apple Watch or could I use my existing one?
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It depends on whether or not you have a GPS only version or a GPS+Cellular version of the watch.
As long as your paired iPhone is nearby, a GPS only version can do everything a GPS+Cellular version does and does not require an additional carrier plan.
If you want to use your watch without the paired iPhone nearby, then you need a GPS+Cellular version which requires an additional data plan. Make sure that you have an eligible cellular service plan with a supported carrier. Your iPhone and Apple Watch must use the same carrier, and you must be within your carrier's network when you set up cellular on your watch. For more info, see:
Set up cellular on Apple Watch - Apple Support
There is typically a monthly $10 charge (in the US) in addition to the iPhone fee for this service (iff you choose to use it).