In answering a telephone call, why do you say "Who is this?" instead of "Who is that?"

In answering a telephone call, why do you say "Who is this?" instead of
"Who is that?"

On the telephone, the person answering it may say "Who is calling?" or
"Who is this?" Why do you say "this" to the caller on the other end of the
phone? Is is wrong to say "Who is that?"