We've been using Office 365 since May 2017 and syncing with company iPhones has generally been smooth. I use the Outlook app for email, and the native iOS Contacts/Calendar apps. Last week we enable MFA. There was a bit of a hiccup getting the mail to flow to mobile devices again, but we seem to be past that. However i just realized that my phone calendar was not updating with new additions made on my PC. Further testing showed the calendar syncing was dead too. Based on a recommendation in post i saw (sorry lost the details), I removed and re-added the Office calendar on my phone. It wiped all of those appointments (no surprise), but will not sync them back. I know i can use the Outlook calendar, but prefer the native app to show other personal calendars. Also, it does not solve the problem of syncing my 3,500 work contacts. Solutions?
NVM, worked it out. After many hours of fussing and fuming, and reading contradictory sync info on MS Azure AD pages, I solved it by logging out of Outlook on my iPhone, deleting the app, and deleting all connections to MS Exchange. I re-established the Exchange source for calendar and contacts, and they have been populating correctly. I have reinstalled Outlook but not yet logged in (real work needs doing). I have also confirmed that edits made to calendar/contacts on my iPhone propagate correctly back to Outlook on my PC.