In movie production, what exactly happens on the other side of phone calls?

There are two things to consider here that relate to your complaint/question.

How are phone conversations filmed?

The answer here is... it depends.

• is the other end of the phone call going to be captured?
  • If so, with audio only or video and audio?
• how is the scene going to be cut?
  • Consideration of this varies by director.

If both ends of the conversation are being recorded, someone will often be reading the dialogue during the scene (either over the phone or out loud) so that the actor can react to it and for the purpose of timing. It may not be the actor actually saying the words, though. Otherwise, they may ask the actor to think through the dialogue in their head as if it were being said, or give them a set amount of time to pause for.

• If the camera is going to stay on the person with only audio being added, this timing is even more important as there won't be a way to pause the "listening" part of the conversation.
• If they are cutting between the speakers visually, this is less important as the scene can be spread out slightly to allow for timing differences.

If only the one end of the conversation is being recorded, as in this example, the director should be conscientious of how the scene will play out and what the actual information conveyed is. If they need to give space for lengthy omitted conversation, they need the actor to pause while filming to give time for them to "listen" on screen or they need to cut away to other people in the room who might be paying attention to what's going on on the phone call.

In some cases, they fail to do this or the take they really liked otherwise has a shorter pause then they like but it's the only take they like so they go with the short pause because... most people don't really think about it.

Why was this gap so short?

There are two parts to this...

First, films occur in what we call "compressed time". Two hours (give or take) can encompass days or years worth of time. To keep the viewers interested and active, they compress unimportant things to keep the pacing of the film going. So, this short window of "listening" time may have been intentional to keep your attention rather than five seconds of watching her listen to the person on the other end of the line... it's why phone calls seldom include greetings or sign-offs... they're implied and unimportant.

Second... perhaps it was enough time. How long does it take to say "Crane was moved to Arkham on suicide watch"? Maybe not one second but probably not much longer than that. Perhaps they have some other shorthand that would make this even shorter and Rachel Dawes simply translates it for us when sharing the information with Bruce Wayne.