You mean you had a test in which the looked-for answer was "the caller was I"? You are quite correct if you preferred "the caller was me". This is what is naturally preferred by most native speakers. I'm afraid you may have fallen victim to one of these bogus grammar myths that people spread about English, in this case that the complement of "be" and other linking verbs should always be in subject case. It's the result of old-fashioned grammarians making up rules about English derived from the grammar of other languages (in particular Latin, which they seem to have held up as an ideal model), rules which have no basis in how people actually use English as a language. Unfortunately, there are still people out there, some of them teaching and testing the language, who believe that people should always say things like "It is I", "Whom did you see" or "to go boldly" rather than their more natural alternative expressions.

If you’re having trouble deciding which one to use take out the other person. You are not likely to say, “Me was the caller,” or “I this is.” !