To see extension and user name on the Mitel 5312 phone display:
You must not be on a call.
The date and time must display.
Press the grey-blue button (icon on button is a phone and page with a list on it).
Press the right-hand bottom button (next to handset).
It will display the extension, ":", user name as configured in the Mitel phone book.
Press the end call button untill the date and time is displayed again.