Speaking as an American, I would likely say "is _this_ Mr. Jack"? I think "is that Mr. Jack" is okay as well, though for some subtle reason, the word "this" sounds more natural. Given that I'm asking a question about somebody that I'm speaking with, "this" communicates a sense of "closeness" in some sense. The word "that" gives me the impression that I'm referring to a 3rd person. As a final note, you typically start the conversation with this question if you aren't certain of who you're speaking to. Given that the majority of people in the United States have personal phones, I think you can expect to know who you're talking to, but in other cases, this question would be used in starting the conversation. When trying to figure out who answered the phone, you can also try the following phrases: - "Am I speaking to Mr. Jack?" - "Speaking?" (this is quite formal and maybe a bit outdated, but it's a shorter way of asking "Who is speaking?", though that phrase might sound a bit harsh.) - "I'm looking for Mr. Jack." If you start the conversation with this, the person will confirm that they are Mr. Jack, or likely give that person the phone or tell you why they aren't available.

The correct phrase to use when starting a phone conversation is: “Good morning. Is that Mr. Jack?” This is preferred because “Is that” is a polite and natural way to confirm the identity of the person on the other end of the call, especially when you cannot see them. “Are you” is more commonly used in face-to-face interactions. ☺️