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This is using Zextras Zimbra add-on for mobile device support.
Shared Contacts folder must be visible in the Zimbra Web Client.
Synchronizing your default Contacts folder is easy and does not require this app to be installed.
If you need to synchronize a shared contacts folder, you can use the CardDAV-Sync Android app. The free version seems to work well on a Samsung S20 phone.
https://play.google.com/store/apps/details?id=org.dmfs.carddav.sync
In the Zimbra Web Client:
ZWC → Contacts (tab) → <your shared contacts folder> (drop list) → Share Contacts Folder
Right-Click the URL → Copy Link Location
Add Account → CardDAV → Enter Zimbra URL and Credentials
Leave
Use SSL
selected.
You can also edit Settings (gear icon) for options like: