Watch number
How do I find the phone number to my watch?
My watch has its own phone number, how do I find out what it is?
Apple Watch Series 5, watchOS 6
Contact your carrier, if it’s not displayed on the carrier’s billing information web portal.
Watch shares the number of the iPhone, with what is called NumberSync.
There is a second number assigned to the Watch for administrative reasons by the cellular provider (all cellular-connected clients must have a number), and that number is not and cannot be used when NumberSync is active.
T-Mobile reportedly does show the number when you manage your account from your paired iPhone or directly from your Watch, but some other carriers do not. You may or will need to use the carrier billing portal.