The long answer is all cellular connected devices such as iPhones, iPads, Apple Watches have a unique "telephone number" assigned to them by the host telephone company. It is the number used by the telco to identify the device when it attaches to the telco's cellular network and is assigned the first time the device connects to the network and an account is established. That number is used ONLY by the telco network and cannot be dialed. As far as I know the only way a user can "change" the number is thorough Settings > General > Erase All Content and Settings although Erase All Network Settings might work but I have not tried it. That does not "change" the number it simply deletes it from the device and a new number will be assigned when the device is linked to a new account.
In any case you would want to be certain the device has been removed from your telco account beforehand. (I won't bore you with my long saga of what can happen if the number is not removed from the telco account first but it isn't fun and can be costly.)
FOOTNOTE: In the unique case of the Apple Watch its "phone number" is the equivalent of a clone of the paired iPhone number.
