Only follow these instructions if the automatic way failed.
To manually sync Haystack contacts to iOS:
Open iPhone Settings
Scroll down and tap Apps
Tap Contacts
Tap Contact Accounts
Tap Add Account
Tap Other
Tap Add CardDav Account
Fill out the CardDAV account details, then tap Next:
Server: https://webdav.thehaystackapp.comUser Name: Your Haystack login email
Password: Your Haystack password
Description: Haystack Contacts
Your iOS device will verify the credentials and save the new account.