Via the settings panel of your iPhone, you can add a subscription to a remote .ics calendar format. I have a Dutch iPhone app that does this from within the app (see the screenshot below, "abonneren op de agenda" means "subscribe to the calendar"), but there must be others too.
I want to mimic this behavior for a project of mine, but I can't find the API to do this with. It looks like it's not a part of EventKit, but because there's no app switching going on when you hit 'subscribe' in the example app I suspect it's also not a url scheme.
Who knows?