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Is the calendar in a shared mailbox or a public folder? If it is in a public folder, you can get an app that will read the public folder and sync it to the phone's calendar. There's this one for iOS, and this one for Android. (Disclaimer, I haven't tested either app, so use at your own risk.) If it is in a shared mailbox, all you need to do is reset the password on the shared mailbox account and add it to the phone like a full account. This only started working recently within the last couple of months or so and it's not officially supported so, again, use at your own risk.