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