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I have shared an iCloud calendar to another user. We are both using iPhones running iOS 9.3.4. When both devices are connected to Wifi a change made to the shared calendar on one device appears immediately and reliably on the other device. However, when both devices only have a connection to the internet over the 3G cellular network, a change made to the shared calendar on one device does not appear on the shared calendar of the other device. Other iCloud services like iMessage, Facetime, iTunes, App Store, and Find my iPhone work over the 3G cellular network. Only the calendar does not sync.

To make matters worse, when a change is made to a shared calendar on a device that is connected to a 3G cellular network but not Wifi that change sometimes takes days or weeks to appear in either user's iCloud.com calendar or on any other device to which that calendar is shared.

  1. How do I make these iPhones sync their shared calendars over the 3G cellular network?
  2. How can I make these iPhones reliably sync their shared calendars when changes are made to appointments while connected over 3G but not Wifi?

Update: One device had 'Calendar' turned off in Settings -> Cellular. (Thanks @fbara.) Now both devices have 'Calendar' turned on in Settings -> Cellular. After some more testing I have determined the following:

  • Changes to the shared calendar reliably sync to iCloud over cellular from either device.
  • Change notifications are reliably sent over cellular to the second device when a change to the shared calendar is made on the first device.
  • The change to the appointment does not appear reliably on the second device when it is connected over cellular even after a notification about that change has been received on that device. This failure of a change to appear seems to happen about once in four changes. None of the failures to show the change have cured after 20 minutes of waiting despite connecting to Wifi again.

The bottom line with my current configuration seems to be that about one in four changes to a shared calendar on the first device are not reflected on the second device when the second device is connected over cellular at the time the change from the first device is synced to iCloud.

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    Have you verified that both iPhones have 'Calendar' turned on in Settings -> Cellular? – fsb Aug 10 '16 at 22:33
  • @fbara Thank you. I have fixed that now, done some more testing, and updated my question with the results. Notifications of changes are now sent reliably (which is good). But some of the changes themselves still get lost between iCloud and the receiving device when it is connected over cellular (which is terrible). – alx9r Aug 10 '16 at 23:09
  • I don't believe changes are immediately sent between devices when on cellular. iOS will batch changes when on cellular to limit the data impact. I don't recall how long it will wait, to see if there are other changes, before communicating the changes over cell. – fsb Aug 11 '16 at 10:30
  • @fbara That's pretty confusing behavior: Over cellular you receive a push notification that an appointment has changed, so you look at that appointment to see the updated appointment, but the change is not there. When it finally is updated with the change you've long forgotten about the change and the notification is cleared. It also doesn't make sense for iOS to wait so long (or indefinitely) to sync once connected to Wifi. I see new changes to appointments appearing while the ones that took place when only connected to cellular are still missing. – alx9r Aug 11 '16 at 12:51

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