How do I set up call greeting?

If you're the workspace owner/admin and want to set up a call greeting, head to Settings, tap on Manage workspace, select Phone numbers (and select the number you want to manage) and then tap on Call greeting. (This feature is currently only available to iOS users, Android users will be able to access this shortly).

Call greeting is a Pro feature, if you're on the Standard plan you'll need to upgrade to get access.