Option 1: Add Outlook Calendar to iPhone with iTunes:
If iTunes is your go-to place to manage your iPhone, this method is perfect for you. Let's see how to add a calendar from Outlook to your iPhone with iTunes.
1) Connect your iPhone to your computer with your USB cable.
2) If iTunes doesn't open automatically, launch it.
3) Click your device icon and open the "Info" tab.
4) Check "Sync Calendars with" and select Outlook from the dropdown menu.
5) Choose to sync all calendars, or only selected calendars using the settings provided.
6) Click Apply to start syncing your calendars with your iPhone.
Consider that iCloud must be disabled for iTunes calendar syncing to work. On your iPhone, go to "Settings" > Your username > "iCloud" and disable iCloud for the Calendars app. Connect the iPhone to your computer, open iTunes and click on your device icon. Click on the Info tab in iTunes and check "Sync Calendars", select your calendar management program and click on the "Sync" button.
Option 2: Add Outlook Calendar to iPhone with iCloud:
You can use iCloud to easily sync data between all your devices which use your Apple ID. Your files are stored online in iCloud, and your devices can access those files if they're connected to the internet and if they have permissions.
If you're using Mac and you are set up with the same Apple ID that you use on your iPhone, this should already be set up for you.
How to Add iCloud Calendar to Outlook:
Please follow the below steps to sync your Outlook calendar with your iPhone using iCloud for Windows:
1) Download and install iCloud for Windows using the link: https://support.apple.com/en-gb/HT204283
2) Enter you Apple ID and password.
3) Select what you'd like to sync (ensure that "Mail, Contacts, Calendar and Tasks" is checked).
4) Click Apply to complete the process.
Now, whenever you add a new calendar event in iCloud, it also appears in Outlook and on your iOS devices - and vice versa.
Option 3: Add Outlook Calendar to iPhone with iPhone Mail App:
You can also add your Outlook account to your iPhone using your iPhone itself. This way, you can choose what Outlook information you would like to sync with your iPhone.
1) Open Settings on your iPhone.
2) Tap "Passwords & Accounts".
3) Tap "Add Account" > Outlook.com
4) Enter your username and password for your Outlook account.
5) When prompted, give the app permissions to sync your data.
6) Tap your newly added Outlook entry in the list of accounts.
7) Choose what information you would like to sync with your Outlook account (ensure that Calendars is enabled).
You may be prompted at this point to decide if you'd like to keep existing calendar events on your iPhone, in which case you may end up with duplicate events on your calendar.