The solution you posted should work fine, but if not, there's also a workaround posted on lovemyecho.com that would allow you to use your Amazon Echo as a 1-way intercom, though it doesn't use the calling system.
Basically, you have to follow these three steps:
1. Record a custom mp3 file
Basically just record an mp3 of whatever you want to communicate to the people in the other room, ie: "Lunchtime!" or whatever.
2. Upload the mp3 file to Amazon Music
Upload the file in a "Messages Playlist", being sure to use good names: they suggest something like
MSG_something or something along those lines.
3. Send the message to an Echo / Dot
Go to the Alexa app and choose the message you want to send from the "Music / Books" area. Having used the
MSG_something file names and having the files in the playlist should make them easier to find.
Obviously, if you want 2-way communication, this would clearly be impracticable, but the advantage of this system is that if there is a message that you frequently want to send, it doesn't require the person on the other end to "pick up the line."