All of the Alexa devices should be automatically in the contacts and talk to each other via their IPs on the same LAN. I was planning use four more echo dots as an intercom system in the house, that is not going to be happening until the Alexa app can set up Drop-in on any device the app runs on.
https://www.amazon.com/Amazon-Echo-Dot-Portable-Bluetooth-Speaker-with-Alexa-White/dp/B015TJD0Y4

