Here are the steps for iPhone Calendar sync with SmarterMaill.

NOTE: I will use in this example, so you will need to replace with your domain.

On your iPhone do the following:
1. Go to Settings
2. Go to Mail, Contact, Calendars
3. Go to Add Account...
4. Select Other
5. Select Add CalDAV Account
6. Server: (NOTE: The /webdav part will disappear after you tap the User Name field)
7. User Name: enter your full email address (see screen shot for example)
8. Password: enter your password for this email address
9. Description: Leave or change as desired
10. Select Next (NOTE: You will get a message about the certificate not being verified. Just accept the certificate).
11. That's it. You should get a message it verified the account.

Sample iPhone Calendar sync image: