Both "It is I" and "It's me" are commonly used, but they have slightly different levels of formality and grammatical correctness.
"It is I" is considered more formal and grammatically correct. In traditional grammar, "I" is the subject pronoun, and in this sentence, it follows the linking verb "is," so technically it agrees with the subject.
"It's me" is less formal but more commonly used in everyday speech. In this construction, "me" is the object pronoun, and it's colloquially accepted even though it doesn't follow traditional grammatical rules as strictly.
Ultimately, both are widely understood and accepted in casual conversation, but if you're aiming for a more formal or grammatically precise tone, "It is I" would be the preferred choice.
