iPhone 6 upgraded to iOS 10 and now Outlook calendar not syncing
It looks like a minor iOS bug preventing iPhone from working with Outlook data. I think the best way is to wait for a week or two till Apple will fix the problem in one of the upcoming updates.

If you need to solve this issue asap, try Akruto: http://www.akruto.com/sync-iphone-with-outlook/

It works like an Exchange server, so if that wouldn't work that means Apple messed up with Exchange ActiveSync technology in iOS 10.0.1 and you still need to wait for a new iOS release.