While the images are taken from an iPhone, iPad owners can also use these instructions.
kick off the FaceTime app on your iPhone or iPad and selectNew FaceTime.
Enter the contact you want to call and selectFaceTime.

The FaceTime video call should start ringing and your devices camera will activate.
The option to record a video will also appear if your contact manually rejects your FaceTime call.
The video message will begin recording immediately.

snag the redStopicon to finish the recording.
pick the camera icon in the lower-right corner to switch between rear and front-facing cameras.
Press thePlayicon to watch the video message you just recorded, or selectRetaketo record a new video.

When ready, go for the green arrow icon to send the FaceTime voicemail video message.
Select Save in the upper-right corner to save a copy of your video message to your gear.
Why Cant I Send a FaceTime Video Voicemail?

Sometimes theRecord Videobutton will appear faded and wont work.
When this happens, this means the contact either isnt using a compatible unit or operating system.
Many messaging apps also have built-in video recording functionality.
Where Are FaceTime Voicemail Video Messages?
Received FaceTime voicemail video messages can be found under missed calls within the FaceTime app.
Tap the Play icon next to Video to play a FaceTime video message.
Received FaceTime voicemail video messages do not appear in the Messages app.
FAQ
The best, most reliable way is to ask them.