You will first set up a safe Place (i.e. home, vacation home, dog sitter, etc.) for your pet using Wi-Fi. Good news is, you can have multiple safe Places depending on where your pet spends most of their time. When your pet’s Whistle is connected to Wi-Fi, it is in power save mode (using very little battery). If your pet is out of Wi-Fi, the Whistle 3 will use AT&T's cellular network to communicate.

Whistle will then notify you each time your pet leaves and returns to their Place. Once your pet is beyond Wi-Fi range, the device uses cellular and GPS technology to allow you to enable tracking anywhere in the U.S. Whistle is the most advanced pet tracker since it uses GPS, Wi-Fi and cellular technology on AT&T (America's largest network) to locate your pet 24/7. 

See these articles for more information on cellular coverage and subscription.