Both forms are used. The first feels abbreviated and casual; you'd use it when you know the other person well and expect them to immediately recognise who you are. The second feels slightly less casual and might (though not necessarily) be followed by a full name and reminder/explanation of who you are.
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Mr. TomThe first is more natural to me, but the second is also heard. To me, the second suggests less familiarity between the speaker and (in this case) Tom.
What about the person on the other end? I mean, which one of the following is better.
Is that Tom?
or
Is this Tom?