I agree with the last comment (Roar) that the most logical way to answer is to say 'That's me'. While either 'she' or 'her', or both, may be correct grammatically, its strange to refer to yourself that way. I always thought 'me' is how you refer to yourself and 'she' or 'her' are the way other people refer to you. If someone asks 'who's that' you say 'its me'. Whether you are answering that you are the one speaking or whether you are equating yourself with who the caller is asking for, is a moot point. Because as the one answering you can interpret the question as you like.