"It is I," and "It am I," are both stiff for the same reason; they are illiterate, for both attempt to mix first and third person pronouns and "to be" verbs.

Another way to answer the question, only this time with the word "me," would be with an appropriate preposition in front of it. For example, one could say, "This is the voice of me." Or if someone asked whose picture this is, one could answer, "It is an image of me."

One could drop the use of pronouns altogether and say, "It was my knocking you heard. May I come in?" Clearly, all the person on the other side of the door just needs to hear is the knocker's voice to know who it is.

Neither is right. "It is ..." is said in the third person, and I, being in the first person, needs to be used with "...am...."

So, the correct way to answer the question, "Who is it?" is to say, "I am who it is." This uses the second pronoun "who" to allow the third person to be used in answer.