The two major players in LTE-connected smartwatch platforms are Apple, with the Apple Watch running watchOS, and a broader vendor ecosystem reliant on the Google Wear.
Smartwatches that include an LTE radio connect to cellular networks automatically. These devices use apps and receive and send messages, even if your phone is far away. In addition to requiring an LTE radio, a smartwatch must connect to the same carrier as the phone. Because the radio, antenna, and battery are smaller in a smartwatch than in a smartphone, you may find that a wrist device doesn't perform as well in marginal cellular connections.
Carriers generally provision LTE-capable smartwatches with a separate data plan and a dedicated phone number, which subordinates to the primary phone number for your account. When someone calls your smartphone, your smartwatch may also ring, and you can place or accept voice calls through its onboard microphone and speakers.