Of course, you can't leave video messages when calling contacts who don't have the Duo app. The videos can only be accessed within the application -- the recipient only needs to tap your icon and then tap "Call now" to call you back. Google says those messages are end-to-end encrypted and will disappear 24 hours after being watched, but adding them to "favorites" downloads and saves them locally. You can start blasting video messages to friends very, very soon: the feature is rolling out on iOS and Android today and will be on everyone's phones (well, everyone who uses it anyway) in the next few days.