Both are correct, but if you say, “It is I” people will know that you have learnt your English from books — old grammar books! When somebody asks, “Who is it” and you reply, “It’s I”, he/she may laugh. The individual will think that you are trying to be pompous. Nobody really says, “It’s I” these days; it’s considered old fashioned. Nowadays, the standard response is, “It’s me!”, or just “Me!”.