Iphone Native Calender App Stop syncing the Calender Events

Hi @Dobbugottu, Venkata Rajendra Prasad (BIC USA) 

Thank you for posting your question in the Microsoft Q&A forum. 

If Outlook mobile works, your Microsoft 365 / Exchange account and mailbox are fine. The issue is usually with how the account is configured in iOS Mail/Calendar (authentication or sync settings). 

When you see “Settings wants to use microsoftonline.com to sign in”, that’s expected. Microsoft 365 uses modern OAuth authentication via the Microsoft identity platform. The flow should: 

-Redirect to your company login page. 

-Accept MFA. 

-Return to Settings automatically. 

If it loops or fails, try the following: 

1.Make sure calendar sync is enabled  

-Go to Settings > Apps > Calendar > Calendar Accounts > Your Exchange account 

-Make sure the toggles are ON for:

• Mail  
• Calendars  
• Contacts (optional)  

-If Calendars is OFF, events will not appear. 

2.Remove and re‑add the Exchange account  

If the calendar still does not appear:  

-Go to Settings > Apps > Calendar > Calendar Accounts > Your Exchange account 

-Delete the account.  

-Restart the iPhone.  

-Add the account again using Microsoft Exchange  

-Complete the sign‑in at microsoftonline.com  

This forces a fresh Modern Auth token.  

3.Set the correct default calendar  

Go to Settings > Apps > Calendar > Default Calendar > Select the Exchange calendar, not iCloud.  

I hope this helps. 

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Follow these steps to restore calendar sync from the company/Exchange account into the iPhone native Calendar app:

1. Confirm the account type on iPhone
   • Go to Settings > Mail > Accounts.
   • Select the company / Microsoft 365 / Exchange account.
   • Ensure it is configured as Exchange / Microsoft 365 / Exchange ActiveSync, not as IMAP or POP.
   • If it is IMAP/POP, remove it and re-add it as an Exchange/Work/School account so that calendar and contacts can sync. IMAP/POP can only sync email, not calendar or contacts.
2. Make sure Calendar is enabled for the account
   • In Settings > Mail > Accounts > [your work account], verify that Calendars is turned On.
   • Open the Calendar app.
   • Tap Calendars at the bottom.
   • Ensure the checkbox for the work/Exchange calendar is selected.
3. Complete the Microsoft sign-in prompt
   • When seeing "Settings wants to use microsoftonline.com to sign in", complete the sign-in with the correct work account credentials and any required MFA.
   • If sign-in fails or loops, remove the account from Settings > Mail > Accounts, then add it again as an Exchange / Microsoft 365 account and complete the sign-in flow.
4. Verify the account syncs correctly via Exchange ActiveSync
   • After re-adding the account as Exchange, wait a few minutes and check whether events appear in the native Calendar.
   • If the mailbox is on-premises Exchange, confirm with the Exchange admin that Exchange ActiveSync is enabled for the mailbox and that the server is up to date (Exchange 2013 requires current cumulative updates for reliable calendar sync).
5. If calendar still does not sync but Outlook mobile works
   • This usually indicates a problem with the native app’s connection or with the device’s compliance with admin policies, not with the mailbox itself.
   • Check with the email/Exchange administrator whether any mobile device or security policies block the built-in iOS Mail/Calendar app while allowing Outlook mobile.
   • If the device must meet specific security requirements (encryption, passcode, etc.), ensure those are satisfied, then remove and re-add the account.
6. As a cross-check, verify calendar sync in Outlook mobile
   • In the Outlook app, go to Settings > [work account] and confirm that Calendar is enabled and events are visible there.
   • If Outlook mobile also stops syncing at any point, follow the Outlook mobile troubleshooting steps: reset the account, or remove and re-add the account in Outlook.

If, after these steps, the native Calendar app still does not show events while Outlook mobile continues to work, use Outlook mobile as the supported client and escalate to the Exchange/Microsoft 365 admin to review ActiveSync/device policies and server-side logs.

References:

• Can't sync calendar and contacts with my phone or tablet
• Calendar items aren't synced when you use Exchange ActiveSync on a mobile device
• This device doesn’t meet the security requirements set by your email administrator
• Troubleshoot Outlook mobile issues