Why in a phone call we use "this is" instead of "I am"?

Why is the right form

Hello, this is yzT from english.stackechange.com.

instead of

Hello, I’m yzT from english.stackexchange.com.

I guess non-natives tend to use the latter due to literal translation, but why do natives use the former? You are identifying yourself, telling who you are; that’s why I don’t understand why the latter is wrong.

"I am" refers to my self, as a being; I was (existed) before I started the call. "This" refers to the caller, who was not the caller before he called. Actually, "this" could also refer to a recipient, or any other 'telecommunicative-endpoint' (such as a radio operator). Note that "this" covers situations where "I" would not be appropriate. For example:

"This is the operator."
"This is the police."
"This is reception, how may I direct your call?"
"This is the USS Intrepid."

Also note that in most German-speaking countries, "here" is often used ("Smith here."). (Where else would I be, if not 'here'?)