Is there an option to use ISO calendar?

What kind of option are you asking about for an iOS calendar or ISO? Here is an explanation of the difference between the two. https://en.wikipedia.org/wiki/ISO_week_date

I do not see a setting for an iSO calendar on the iPhone, but there may be a calendar app in the App Store that will provide you what you are looking for. You can search the App Store for one if you wish.