If you are talking about ML/EL this refers to the Mitel SX-200. The SX-200 ML/EL is long discontinued and no further changes to its software will be completed. The ML/EL/ELx has been discontinued for almost ten+ years. It was replaced by the SX-200 ICP. The SX-200 ICP supports analog, digital and IP phones. The SX-200 ICP does support notification of emergency numbers being dialed. From the console, it indicates the number dialed (911, 999, ANY other emergency number you identify in programming), the extension name and number that dialed the emergency call, as well as the time and date. The system has always supported this feature. I'm not aware of any Mitel system ever looking for busy trunks and dropping a call to make an emergency call. I'm aware other systems from other manufactures are able to do this - however I've never seen any Mitel system drop a call from a busy trunk group to facilitate an emergency call.
Going forward a Mitel Communications Director (MCD) is the ideal replacement for the SX-200 and SX-200 ICP. The MCD system supports console-based notification of emergency numbers being dialed, plus e-mail notification (with the same details as the console), and also emergency notification to one (or more) IP phones (i.e. the same information present to the console is also presented to phones programmed to receive emergency dialed information). In addition SNMP is supported for emergency calls as well. You can also play a funny ring on a ONS device (analog phone/cordless phone) if an emergency number is dialed on the property, however since it's analog, there is no data on the screen to understand where or why the emergency call was placed - thus you would need to go to a console or Mitel IP phone with a display.
Hope that helps!