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Any CalDAV connector for Android will do the job. We recommend CalDAV-Sync: https://play.google.com/store/apps/details?id=org.dmfs.calda...

Once we have CardDAV available, then we'll be recommending CardDAV-Sync.

Any option for ios?

iOS supports CalDAV natively. Just follow the instructions at https://www.fastmail.com/help/clients/iphone.html (scroll down for calendar section).