iPhone Calendar Does Not Sync All Entries in Outlook
There are several ways to sync the iPhone Calendar app to Microsoft Outlook. Most of these involve routing calendar entries through another service, like Google Calendar or iCloud. If some of your entries aren't syncing, try syncing directly to Outlook 365 from your iPhone. You can sync your calendar without necessarily syncing email, contacts or reminders. If only past events aren't syncing to your iPhone, enable the option to sync past events in the iPhone's Settings.
1 Syncing With Office 365
To sync your iPhone Calendar app with Microsoft Office 365, make sure your iPhone has access to the Internet, and then launch the "Settings" app, select "Mail, Contacts, Calendars," and then select "Outlook." Enter your email address and password to see a list of options, including syncing calendars. Turn the "Calendar" option to the "On" position, and then wait a few minutes for all of your Outlook 365 calendar entries to be synced with the Calendar app on the iPhone.
2 Syncing Past Events
When you sync the iPhone's Calendar with other calendars, past events are not uploaded to the iPhone if you have not specified that they should be. You can specify how far back the iPhone should sync events in the iPhone's Settings. After selecting "Mail, Contacts, Calendars," scroll down to the Calendar section, and then select "Sync." This gives you the options for syncing all past events, ranging from two weeks to six months or all past events.