Walkie talkie
Do I need cellular service on my Apple Watch to be able to use the walkie talkie.
Apple Watch Series 4, watchOS 8
No, but without it you need to have your suitable iPhone with you that has a cellular plan, ie to be able to use Facetime. To use Walkie-Talkie, you and your friend both need to have an Apple Watch Series 1 or later with watchOS 5.3 or later. You also both need to set up the FaceTime app on your iPhone with iOS 12.4 and be able to make and receive FaceTime audio calls. The Walkie-Talkie app isn't available in all countries or regions.
Use Walkie-Talkie on your Apple Watch – Apple Support