Old outlook calendar invites

Old events that have been cancelled or I have declined are showing on my iphone, but not on my outlook calendar. If I go in and decline them again sometimes they go away, but they reappear. I also may get an error message " a response to this invite cannot be sent." The calendar invites are from both people who no longer work here and people who work here. Some I have declined or deleted; some the meeting organizer has.

It is driving me crazy and I almost can't use my phone calendar any longer because I don't know which appointments are "real."