To resolve the iPhone Calendar Not Syncing With Outlook issue, first, check the settings on both the Outlook and iPhone calendars to make sure that they are set to sync with each other. If they are not, then change the settings so that they do the sync. 

Alternatively, try resetting the calendar on the iPhone. To do this, go to the Settings app on the iPhone, tap on the General tab, and then scroll down and tap on the Reset button at the bottom of the screen.