Cache Exchange mail on iPhone

Losing contact with the Exchange server should not result in losing contacts unless you restored the phone while out of contact.

A restore from backup media for the Exchange server is the remedy. The information on the devices is simply a copy of the master information kept on the Exchange server.

If I turn on Airplane mode the contacts and calendar entries do not disappear. So they are cached locally. The Exchange Global Address List is NOT cached; that is available only when connected. This makes sense, as in a large company there could be hundreds of thousands of entries.

If your contacts disappeared it is likely that the server was still responding online but had lost access to the contact list, so it "synced" the now empty list in Exchange to the phone.