OP is referring to the ability to tap into any Alexa device, a feature Amazon calls Drop In (https://www.amazon.com/gp/help/customer/display.html?nodeId=...).

The receiving device has to grant permission, and every time you drop-in it plays a sound effect to alert the user they've been dropped-in on.

Google Duo does offers it as well.

I think you're referring to the Duo feature which shows a video of the caller to the person being called prior to answering. Basically the opposite of drop in and of this FaceTime issue.