I have the drop-in function working between Ireland + UK + Germany. The primary requirement for me was to use the Echo Show so that we could have video conferencing without my mother needing to use a keyboard or a remote control ... or indeed, any device. Everyone is happy with the video quality and we think that a lot of the problems of the past were due to my mother not holding the phone up to her ear correctly. Now it's all hands-free and works a treat!
Where I initially got into difficulties (and perhaps some of them would have been avoidable if I had had more time / patience / knowledge) was when trying to configure / control two Alexa Echo Shows using the same Amazon account ... or even the same mobile phone. Basically, Alexa is using the mobile phone's contact list associated to the Amazon account ... so if I want my mother to be able to call someone new, then I need to be able to manage her mobile phone. If my mother wants to allow someone "drop in" privileges then this, too, can also only work if you have access to the correct mobile phone.
As my mother doesn't own a smartphone, I had to buy a new one with dual SIM cards (one Irish and one German) and register and associate those phone numbers with the appropriate Amazon accounts. In order not to complicate things, I set up two new Amazon accounts, one for each device ... which means that the Prime status of my primary Amazon account does not apply to either of those new accounts.
I hope this helps.
Michael.