Setting up and synchronizing Outlook calendars on iPhone

[taherte]

I am running Outlook 2016 on my desktop computer and have multiple email accounts. I would like to have the calendars associated with those email accounts automatically downloaded to my iPhone and kept in sync with my Outlook 2016 calendars. I thought I could do it by installing Outlook for iOS on my iPhone. I downloaded Outlook to the iPhone and set up the mail accounts. They all appear to be working. However, when I click on the calendar tab in iOS I get the message "No Connected Calendars". If I select the option to add an account I get a message that it already exists. What do I have to do to get the calendars set up and connected? Or does that only work with Outlook.com? If I go into Settings/Calendar/Accounts the only options there are for mail and notes. No toggle switch for Calendar.