Figure 11: Email Notification; Figure 12: System Events (Emergency Calls) - Grandstream Networks UCM6 Series Manual

The call which was dropped is the one occupying line1 since it's the line with highest priority. This behavior
is mainly because we had the option "Disable Hunt on Busy" deactivated, if activated, then the UCM will
not hunt on busy lines to allow emergency calls to go through.
After the call has been initiated we can see also that UCM is sending notification calls to the selected users
which could be personal for first aid at the company, and they will hear the custom prompt configured on
911 emergency number, if non-specified then the default prompt will be played.
The system admin, did receive on the fly the following Email notification.

Figure 11: Email Notification

Also, events notifications can be seen on the UCM's web UI under « Maintenance → System Events ».

Figure 12: System Events (Emergency Calls)

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UCM6xxx Emergency Calls Guide