With the release of IOS 11, the instructions for adding an Office 365 account to the default iPhone App have changed slightly.  Here are the revised instructions.

Step 1 – Click on Settings from your iPhone Home Screen

Step 2 – Click on Accounts & Passwords

 

Step 3 – Click on Add Account

Step 4 – Click on Exchange

Step 5 – Enter your Email Address, Description and click on Next

Step 6 – Click on Sign In

Step 7 – Enter your email address and Office 365 Password

Step 8 – Click on Save Password and then Accept

Step 9 – Click on Save