4G watches
They carry their own nano SIM with data and calls (VoLTE), so they work even when the child or grandparent is far from your phone. The SIM plan is paid to the carrier.
Health watches
The S-F3 and S-ET488 connect to your phone by Bluetooth and need no SIM. They must be near the phone to sync to the app.
The app
Each watch uses the manufacturer's free app for iPhone and Android. There's no monthly fee for the app.
How accurate the location is
Outdoors, GPS locates the watch accurately. Indoors it uses nearby WiFi networks and cell towers, so the location is approximate.
Health readings
Blood pressure, heart rate, oxygen and ECG are for reference and tracking. These watches are not medical devices and don't replace a doctor's visit.
SOS button
You register the emergency numbers in the app. When the button is held for about 3 seconds, the watch calls those numbers and alerts the app.