By default a static message will display on a monitor with

Get your loadout from the ORBAT
Report to your commanding officer
Modify gear when needed
Get ready for mission start

When an admin presses the 'Countdown' button in the UNITAF Admin Menu, the message will alternate with a countdown timer

Mission start in 5 minutes
Mission start in 4 minutes
Mission start in 3 minutes
Mission start in 2 minutes
Mission start in 1 minute
Mission start in 30 seconds

And at 0 it will show

Mission starting, report at briefing NOW!