To get the true ending, you have to complete 72-Hour Mode and unlock Overtime Mode and then also complete Overtime mode.(which unlocks Infinity Mode)
To unlock Overtime Mode.
You have to do all the case files AND talk to Isabella at 10am on the last day, then be at the Helipad at noon. If you just complete the case files, and don't talk to Isabella when she calls you after the special forces withdraw on the last day, you'll get Ending C instead of the Ending A.(and won't unlock Overtime Mode.)