No personal knowledge of them but a couple would be sabre/dav and radicale:

• https://github.com/sabre-io/dav

• https://sabre.io/

• https://github.com/Kozea/Radicale

• https://radicale.org/v3.html

edit: and for Android phones the DAVx⁵ is a great caldav and carddav client
https://f-droid.org/packages/at.bitfire.davdroid/