Open the Outlook app on your iPhone
Sign in with your Outlook account
Tap your profile icon or initials
Tap the gear icon for Settings
Tap your email account
Turn on Calendar sync
Open the iPhone Settings app
Tap Apps
Tap Calendar
Tap Calendar Accounts
Tap Add Account
Tap Microsoft Exchange or Outlook.com
Sign in with your Outlook account
Turn on Calendars
Open the Calendar app on your iPhone
Check that your Outlook calendar appears
Refresh the Calendar app if needed
Make sure Outlook has permission to access Calendars
Go to iPhone Settings
Tap Privacy & Security
Tap Calendars
Enable Outlook access
Update the Outlook app if syncing does not work
Restart your iPhone if the calendar still does not sync