By convention, complementary words like here/there and this/that are relative in their usage, i.e., their meaning depends on which side of the conversation they fall into.
- To introduce oneself on a telephone, it is common to say "This is John."
- On the other hand, to ask someone on the other end of the line about their identity, the usage is still "Who's this?" or something like "Is this John?" when in doubt.
- Combine the above two into a single statement, and hell breaks loose! "Hi! This is John. Who's this?" is just so ridiculous a statement!
See also: how-to-answer-is-this-john …
How did the word this come to be used to refer to the both sides of a telephonic conversation?