A Bluetooth smartwatch is a wrist computer that talks to your phone over Bluetooth, not a fashion watch with no radio. US shoppers split flagship GPS models from budget watches that advertise Bluetooth calling on the speaker. Pairing only works if the watch OS matches your phone. LTE versions add a carrier plan; Bluetooth-only versions need the phone nearby for calls and messages.
Confirm water rating, heart-rate sensors, and whether calling uses the watch mic or only phone relay. Fashion quartz watches will not show notifications.