Thought it would maybe be helpful to share my recent experience.
We have three Echo dots in the house -- living room and in both kid's bedrooms. Also have an Alexa enabled Firestick in the TV. So the idea of being able to drop in on the kids to tell them to come for dinner, etc, is VERY appealing. No more yelling at the top of my lungs to get their attention and draw them away from their gadgets.
But it was never appearing as a feature until, last week, I spotted a short-lived, YouTube video that explained how to activate the feature.
Essentially I learned that if I changed my address to the USA then, as if by magic, the features for calling and messaging appeared in the app after re-starting. I tested the room to room calling or calling from the app to a specific Echo dot and all worked just fine. Sorted!
But there was a fly in the ointment. Most of my services are through the UK Amazon site including the Echo Music deal where you can have all the music you want on one echo for 3.99 a month. I'd just begun this subscription (after cancelling my old LoveFilm DVD subscription).
Annoyingly, after testing all the Echo messaging features were up and running, I then discoved that the Echo Music subscription was now AWOL. I did get it back but it required a call to customer support and changing back my account info to the UK.
So everything does work as described in the numerous videos explaining how good it is to have... but if you subscribe to rolling IP based services such as Amazon Prime TV or Amazon Music, I suspect these will break.
So for me it's back to square one and waiting for Amazon to get around to enabling the service for the pesky Brits...
Cheers
Ralph