Microsoft is rolling out a feature update for shared calendars so that they can sync to mobile phones. This update will enable all your shared calendars to sync to mobile phones for any permission level (view, edit, or delegate permissions).

How does this affect you?
New shared calendar invitations that are accepted (after the changes are rolled out), will sync to mobile phones. Existing shared calendars will not be upgraded as part of this change. They will continue to sync only to Outlook on the web, Windows, and Mac. In addition, the shared calendar will only sync to mobile phones if: – The calendar is shared between users who are both hosted in Exchange Online – The calendar is accepted from a sharing invitation using Outlook on the web, iOS, or Android.