The caliphate is an islamic belief and goal. Not an exlusive domain of ISIS.
the concept of Caliphate is more complex than you might think However,ISIS is not an Islamic Caliphate recognized by islamic sources
with ISIS i have read many of muslim scholars view and Majority of them believe ISLS is a deviant group according this valid narration :
((The Prophet said black flags will appear from the lands of the east, and they shall fight and they shall kill you like no other nation has been killed.
When you see them - then give them allegiance - even if you have to crawl over snow to get to them, because they shall have the khalifah of Allaah - the Mahdi in their midst.))