Danny 
If anyone saw my post about trying to drop in on my Amazon Fire TV, well I finally got it working, but the solution was totally (in my view) counter intuitive.
Seems all I had to do was unmute the built in microphone using the switch on the bottom of the TV.
Why it wouldn't allow a drop in with a muted microphone baffles me, but I guess why would you drop in with no audio?
Anyway it's working now and I'm celebrating with a bottle of Bank's Amber bitter.