The Nextcloud contacts app is a user interface for Nextcloud's CardDAV
server. Easily sync contacts from various devices with your Nextcloud
and edit them online.
 * Integration with other Nextcloud apps! Currently Mail and Calendar -
   more to come.
 * Never forget a birthday! You can sync birthdays and other recurring
   events with your Nextcloud Calendar.
 * Sharing of Adressbooks! You want to share your contacts with your
   friends or coworkers? No problem!
 * We're not reinventing the wheel! Based on the great and open SabreDAV
   library.

WWW: https://apps.nextcloud.com/apps/contacts