I don't know if anyone else has achieved this yet, but I have been trying for months and months and have finally got CardDAV working on Windows Phone 8.1.
- Add an iCloud account.
- Set the email address to your fastmail email address, but add "+Default" after the name part so you have, for example "name+Default@fastmail.com".
- Put in your password (if you're not aware you can create an alternate login for this so you don't have to use your master password).
- Tap "sign in".
- Tap the newly created iCloud account.
- Change the account name to something meaningful.
- Make sure Contacts is ticked to sync.
- Untick sync Calendar.
- Tap "advanced settings" at the bottom of the page.
- Edit the "Contacts server (CardDAV)" to:
"carddav.messagingengine.com:443/dav/addressbooks/user/username@fastmail.com/Default"
Obviously changing "username@fastmail.com" as appropriate. Note the missing "https://" and the added port number.
- Tap the tick to save your changes.
- Your Fastmail contacts should now sync.
Note that you can't use the same account instance on your phone for both Calendar and Contacts because the usernames are different.
https://beta.fastmail.com/help/techn...44101#contacts