It’s simple. When you pair your Apple Watch Series 3 (GPS + Cellular) with your iPhone, you will be prompted to set up cellular service with your provider. Or you can set up service later using the Watch app on your iPhone.
Apple Watch shares the same number as your iPhone. When you receive a phone call and your Apple Watch is set up and powered on, it rings whether or not your iPhone is nearby.
Apple Watch Series 3 (GPS) uses the network connection of your nearby iPhone to send and receive text messages, answer phone calls, and receive notifications. The built-in GPS works independently of your iPhone for distance, pace, and route mapping in workouts. And you can do many other things independent of your iPhone, like measure your heart rate, play locally stored music, and pay with Apple Pay.
Apple Watch Series 3 (GPS + Cellular) features built-in cellular, giving you the freedom to stay fully connected with just your Apple Watch. You can make and receive phone calls, reply to messages, and get notifications even when you don’t have your iPhone with you.
While service plans vary by carrier, many offer free or special trials for Apple Watch.