At present, the walkie-talkie feature can only be activated in the walkie-talkie app. However, following the update of watchOS 5.1.2, you can turn the walkie-talkie function on or off directly from the Control Center. When the walkie-talkie function is turned on, the icon turns yellow and turns gray when turned off. As with other Control Center icons, you can also enter the edit mode by a long press to change the icon position.
Introduced at WWDC this June, Walkie-Talkie is a feature that allows you to get in touch with someone just a tap. You can also add friends, then control when you want to talk. This is honestly way easier than call someone, just raise your wrist, then you should be able to connect someone at any time, but only limited to Apple Watches with watchOS 5.