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)
GUID: 6x3331a8-as12-ea2x-4ou1-ndeb3ct4wa686
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.
eg:
https://username@yourserver.somewhere.local:8443/principals/__uids__/6x3331a8-as12-ea2x-4ou1-ndeb3ct4wa686