I agree that we need some documentation of some essentials...like stand down orders and all orders sent through the chain of command. That is a must!
I do think that hearsay should be allowed though, as the content of phone conversations would be a hard thing to have a record of, etc.
Your point is a good one and probably why the emails are so important. They back up what is being said in testimony.
There may or may not be recordings of phone calls BUT there will be records of calls made and how long those calls lasted. In the age of tech. Expect more emails or text messages.