Cellular service on my watch - find phone number

The Apple Watch phone number, should be your iPhone's phone number.

Some carriers associate an internal number for accounting purposes. Since each phone number is unique, many cellular companies use the phone number as the account number, so if there is an account number associated with your Apple Watch on your cellular bills it might be the cellular companies internal number.

You won't find that number anywhere on the iPhone or the Watch. As BobHarris has said, that number is an accounting number and is only used for billing purposes. If you transferred the cellular account from one watch to another, it would have transferred correctly if the account on the Watch shows it being active. I've switched over mine for every watch I've had since the first cellular one and never experienced a problem. If you are worried, have someone call your phone and see if the Watch rings. And then, turn off the paired iPhone and call you phone number and it should ring on the Watch. Make sure that the Watch connects to cellular before you test that.