Business Mail service makes it possible to set up calendar synchronizations separately on devices that support CalDAV configuration.
1) Open up Settings on your iPhone
2) Navigate to Passwords and Accounts
3) Tap on Add Account
4) Tap on Other
5) Select Add CalDAV account
6) Enter the configurations as mentioned below:
a) Server : dav.flockmail.com
b) Username : Your email address
c) Password : Your mailbox's password
d) Description : Set as you wish
7) Make sure that Calendars is turned on and tap on Save
You will now have successfully set up your Business calendar via CalDAV on your iPhone.