How to recover Outlook 2013 calendar items deleted in synched Android phone

I recently purchased a new Android phone. In the old phone, I deleted all of the calendar items thinking they would just be deleted from that phone. However, the phone was synched to my MS Office 2013 Outlook calendar, so all these appointments were deleted from Outlook as well. I looked in the Deleted folder in Outlook and they are not there. The Recoverable Items retrieval did not find them either. How can I retrieve/restore these many deleted appointments?

Thanks, Paul