To access the shared resource/location calendar on a iCal server from an iPhone via the caldav protocol you’ll need the GUID of the shared calendar.
First, find the GUID of calendar on the server using the
calendarserver_manage_principals command, eg
sudo calendarserver_manage_principals --search shared
1 matches found:
your shared cal (Resource)
Record name(s): 6x3331a8-as12-ea2x-4ou1-ndeb3ct4wa686
Then on the iPhone (assuming the device is unmanaged), create a new CalDAV calendar account & fill in the correct server, username & password field & hit next.
Assuming everything went ok, you should be back on the “Mail, Contacts, Calendars” page, select the new calendar you just created & advanced settings.
Edit the “Account URL”, replacing the UID on the end of the url with the GUID of the shared calendar you noted down on the first step.