Walkie-Talkie works over an internet connection. If you don’t have the cellular connection Apple Watch, you need to be on Wi-Fi and have your iPhone nearby —the same as data worked before.
These instructions apply to Apple Watches with watchOS 5.3 or newer and iPhones with iOS 12.4 or later.
The Walkie-Talkie app comes with watchOS 5.3 and later. You don’t need anything other than an iPhone to get started. Both parties using Walkie-Talkie need to set up FaceTime on their iPhones (on iOS 12.4 or later) and be able to make and receive audio FaceTime calls.