Apple Watch
Can you have an Apple Watch on a family plan with its own separate number than the iPhone it was connected to?
Apple Watch Series 3, watchOS 7
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I believe that the correct answer to your question is "No".
My understanding is this...
Although a watch seems to have its own number, the number is simply used for billing purposes through the carrier.
The watch is actually cloning the phone number of the iPhone. People call one number, both devices will ring.