Syncing your Calendar onto your iPhone is a bit of a different process than syncing with a calendar system on your computer. You would use the same link, but subscribing is a bit different.
Log into Sprout Studio on your iPhone.
Go to Settings > Account> Users > Edit User.
Go to the Calendars tab.
Click and copy the Leads URL you're looking to sync (keep in mind, this option only shows up if you aren't using Google Calendar).
Now that you have the calendar URL, you need to add it to an account on your iPhone.
Go into the Settings on your iPhone.
Go to Calendars > Accounts.
Click Add Account at the bottom of the screen.
Choose the Other option.
Click on Add Subscribed Calendar.
Paste in your URL here and click Next so it syncs with your phone.
That's it! Now, you'd need to repeat the process with the Shoots and Unattached URL's in your Sprout Studio account to add them to your iPhone.