KAddressBook

It supports large variety of services, including NextCloud, Kolab, Google Contacts, Microsoft Exchange (EWS) or any standard CalDAV server.

Features:

Powerful search - KAddressBook has configurable filters and powerful search capabilities. Duplicate detection and merging - KAddressBook can find duplicated contacts from multiple sources and can merge them into a single contact. Good standard support - KAddressBook can import and export from and to nearly every address book standard. LDAP integration - Can connect to multiple LDAP servers, which can then be used for contact autocompletion when composing emails in KMail. Integration - Integrates with other Kontact components, e.g. exporting birthday reminders to KOrganizer QR codes - KAddressBook can display a QR code for each contact to quickly scan it into your phone or send to someone.