CardDAV Account

This may help you --> Set up mail, contacts, and calendar accounts on iPhone - Apple Support (IN)

1. Set up a contacts account
   1. Go to Settings  > Contacts > Accounts > Add Account.
   2. Tap Other, tap Add LDAP Account or Add CardDAV Account (if your organization supports it), then enter the server and account information.
2. Set up a calendar account
   1. Go to Settings  > Calendar > Accounts > Add Account.
   2. Add a calendar account: Tap Other, tap Add CalDAV Account, then enter your server and account information.

CardDav Contact or Calendar?

You may get better and prompt responses if you may kindly provide the iPhone Model that you have and the iOS version if it is known to you and re-post.

With the limited information that you have provided, It is hard to even guess the problems faced by you. 

You may get faster and better responses to your query by --> Writing an effective Apple Support Commun… - Apple ...

Please re-post with detail so that someone else can help.