Yes and no. It does have it's own phone number for the SIM card, but you don't need to treat it as having a second line. (On ATT, it's an extra $10 charge like a tablet) Once you turn on Auto Call Forwarding, it's best to just pretend the Gear S phone number doesn't exist. Technically, when you move out of BT range the remote connection and call forwarding are established they use that number to make the connection between the two devices. And you can set up your messages from the Gear S to show your mobile number instead of the Gear S number. But the people trying to contact you only need your existing mobile number, the remote connection and call forwarding reroutes the stuff coming into your phone over to the Gear S.
However, if your phone is off or for some other reason the system cannot establish the remote connection and call forwarding, then the Gear S goes into stand alone mode and acts like a second phone and the calls would have to be sent directly to the Gear S number. But in my experience that's a rarity. It usually occurs when I'm in the car as I move out of BT range. It seems like poorer signal inside the metal cage of a car body or the moving between cell towers makes it harder to establish the remote connection. But once I arrive at my destination and am out of the car a couple minutes, the connection gets made. There is an automatic one minute delay as you move out of BT range before it attempts to start the remote connection to minimize shifting back and forth if you are right on the edge of the BT zone.