Sharing number between iPhone and Apple Watch
I have an iPhone. If I purchase an Apple Watch 5, can it share the same number as my iPhone?
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That is how it works. The iPhone shares its phone number with the Apple Watch. There is no other option.
Cellular carriers that support the Apple Watch charge a monthly fee to share the phone number. In the U.S. it is $10 per month added to the iPhone bill.
You cannot have a separate number. You can not have a separate carrier.
iPhone Cellular Carriers that support the Apple Watch, which models, the radio frequencies each model uses, and the country where they offer service:
https://www.apple.com/watch/cellular
You must have an iPhone 6s, SE, 7, or newer running iOS 13