Is there?

Is (someone) there?

Is someone available to talk on the phone? A: "Hey, is Jen there?" B: "May I ask who's calling?"
Farlex Dictionary of Idioms.

Is someone there?

a way of requesting to talk to someone in particular over the telephone. (This is not just a request to find out where someone is. The someone is usually a person's name.) Tom: Hello? Mary: Hello. Is Bill there? Tom: No. Can I take a message? Tom: Hello? Mary: Hello. Is Tom there? Tom: Speaking.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.
See also:
  • on one's
  • on someone's
  • pillow-biter
  • run someone out of town
  • save someone's skin
  • (Have you) been OK?
  • a big ask
  • (something) blows
  • do someone or something justice
  • out of one's
References in classic literature
"'Who is there?' cried the Doctor, putting his head out of his bedroom window.
The Happy Prince and Other Tales