Elaborate?
It doesn't really prioritize them. Your 911 routes would have FRL=0 so anyone can call, but its not like CM is going to kick someone off an active call to free up a trunk for your 911 call if you mean prioritize that way, like in the military MLPP sense.
When you're talking about E911, are you referring to service provider level e911 or something more local/on campus for your client? Because there are 3rd party products that tie into AES, get the crisis alert, and map out your IP network/dataswitches/ports and if you have your cabling down to a science and know what room/cube is on which switch port, e911 solutions on premise can alert building security.
At the most basic level, a dialed string (like 911) is defined as a call type of "alrt" which triggers crisis alert keys, and routes the same as any other call, but should be requiring a much lower permission level to be permitted to make the call.
