https://www.cnbc.com/2017/09/20/apple-watch-series-3-loses-connection-fix-coming-apple-says.html

In a review of the new Apple Watch Series 3 with GPS and cellular connectivity, The Verge noticed that the watch often didn't connect to networks properly.

This resulted in an occasional inability to place or receive phone calls and text messages, or access Siri. Since that's a primary selling point of the cellular-connected Apple Watch Series 3, it's a pretty big deal.