Synching iPhone with internet calendar (.ics)
I found the answer to my particular issue.  The reason that my iPhone could not subscribe to a published Outlook .ics calendar was because the MIME type of the .ics file was wrong.  As recommended in this brilliant post by Joe Bradford, I went into IIS, changed the MIME type from application/octet-stream to text/calendar, and iPhone could connect and subscribe without further issue. Success! 

PS: I did not run into the second issue that Joe chased down, which was having to remove the Byte Order Mark (BOM) from the .ics file via a hexadecimal editor.