Watch cellular plan
Is it possible to get a stand alone cellular plan for an Apple Watch to work with a company iPhone without being on the same plan as your phone?
Apple Watch Series 4, watchOS 7
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When pairing to your iPhone, you share your iPhone's phone number. As Dah•veed says, that adds about $10 per month to your iPhone cellular bill.
You can use the Apple Watch Family Plan to give the Apple Watch its own phone number, HOWEVER, under this plan, the Apple Watch has fewer features, as it is assumed to be a dependent child or elderly parent, and the iPhone owner is managing everything. And because the Apple Watch is not directly paired to the iPhone, the services that depend on accessing personal information on the iPhone are blocked, as dependent children should be be accessing the parent's personal information. This is also a more expensive plan option.
https://www.apple.com/newsroom/2020/09/apple-extends-the-apple-watch-experience-to-the-entire-family
Finally, if your iPhone support Dual SIMs (a physical SIM and a eSIM), you can get a 2nd line for your iPhone, and associated the Apple Watch with the 2nd line. It is still paired to your iPhone, and your iPhone will still ring if a call comes in, but your original phone number will not ring on your iPhone as that number is not paired to the Apple Watch. Again this is a more expensive option
https://support.apple.com/guide/watch/use-apple-watch-with-dual-sim-iphone-apd641d0e7f1/watchos
