Because iCloud calendar for the iPhone is made by Apple, and Outlook calendar is made by Microsoft.......there is no easy straight forward way for the sync to work seamlessly. Its there way of forcing you to choose.
I would suggest using a 3rd party go between.
I would suggest that you choose as well.
As much as you may like to use the iPhone calendar......you will always have an issue with a seamless hassle free calendar Sync, to ANY microsoft product. And vice cersa.
Creating a calendar event on your Outlook calendar, and having it sync to your iPhone calendar is what the iCloud windows control panel will do.
Creating a calendar event on your iPhone calendar, and having it sync to your outlook calendar is where a 3rd party would need to be used.
I would HIGHLY suggest you use a Google Calendar.
WHY? Becasue Google plays well with BOTH Apple and Microsoft. And can be the go between for syncing your calendars.
So basically, your iphone syncs with google.....and outlook syncs with google.
Keeping both up to date, and synced properly.
This is can get complex, but its your BEST option.
This is not a new issue and there will ALWAYS be a DIVIDE between Apple and Microsoft.