Can't open shared .ics file with iPhone

I am trying to sync my iPhone calendars with the calendar on my PC that is in .ics format.  The .ics file is located on a home server that allows both me and my wife to see each other’s calendars and its been working great.  Now, I want to be able to see my calendar from my iPhone 4 (with OS 5.1.1) .. and I’ve tried setting it up with:  settings --> mail, contacts,calendars --> add account --> other --> add subscribed Calendar. 

With every possible config I can think of, I get the same response: "Calendar Subscription -- Unable to verify account information" 

For server name, I’ve tried actual servername, ip address, etc; we do have security on the server but I put in the account name and password … ‘nuthin! “Unable to verify …”

Through the phone's browser, I am able to open up the folder on our server and see the.ics file. The path is xxxx.homeserver.com/webcal/nat_calendar.ics. I've tried sending an invite to myself, opening it in the phone, clicking on the link and I get this error: Cannot Subscribe: Could not subscribe to the calendar "nat_calendar.ics from "myservername".

Seems simple, what am I missing here.

Any ideas?

-- Nat