If you are using the latest version of FaceTime, tap the green icon that resembles a video camera. Then tap Join to accept the call. If you are using an older version of FaceTime, tap Accept to accept the call. If you are not yet ready to be seen on camera, you can cover the front-facing camera on your iPhone or iPad with your thumb or a piece of tape.
If you are using an older version of FaceTime, it will be the icon that resembles a camera with a line through it at the bottom of the screen. If you do not see the panel with the camera icon in it, swipe up from the bottom of the screen to display the menu.
Alternatively, you can enter their name or email address in the search bar at the top. Then tap the icon that resembles a phone receiver next to their name.
You can also place a FaceTime video or audio call to anyone using the Contacts app or from within the Messages app.