If you have a cellular version of the Apple Watch and your carrier supports activating cellular, it uses the current phone number from your iPhone. However, most, if not all, cellular carriers charge a monthly fee for the Watch to have access to the network. In the US, it is $10/month. It is not a new plan, but it is an addition to your current plan at that $10 fee. While the Watch is assigned a number, it is not one used to call the Watch, it is just for billing purposes only. Depending on your location, the costs may differ. Check with your cellular carrier.