Why people introduce themselves on phone by saying "This is John" instead of "I am John"? Which one of "this" and "I" or "he/she" is preferable in different circumstances?
1) Introduction of oneself (unseen): - This is John [speaking]; This is [the person called] John. Here, this is used to locate the person in "space", here, on the telephone.
2) Introduction by another person of a person(seen): - This is John [here, you can see John, and another person is introducing him to you and you are not Harry, for example, in roomful of people, it is used to locate him in space. This is John, "not someone else"].
These uses come under the general heading of deixis in English, which can get rather complicated. This and that are used as deictic pronouns. This notion can be looked up separately. They are used to provide information regarding proximity/distance of speakers to each other in speech. deixis
Now, in order to identify oneself as a physical body, one says, to another person: "I am John." [that is, I'm the person called John. Not Harry or Robert.] "We spoke on the phone, remember?"