Elva's phone number is KL-5-2368. The K and the L are both the number 5 on the phone dial. "555" is an exchange number commonly thought to be reserved by the phone companies for use by TV and movies in order to prevent prank phone calls to real people. In fact, only 555-0100 through 555-0199 are now specifically reserved for fictional use, and the other numbers have been released for actual assignment.
The title of Richard Matheson's original short story is "Long Distance Call". However, as there was already an episode of The Twilight Zone (1959) with this title, Long Distance Call (1961), the title of this episode had to be changed.
Originally scheduled to air on November 22, 1963, it was preempted by John F. Kennedy's assassination. In the alternate timeline featured in Profile in Silver/Button, Button (1986) in which JFK's assassination was prevented, a CBS television announcement is heard: "We will now return to our regular programming" and the theme of The Twilight Zone (1959) is played, a reference to the intended broadcast date of this episode.
This episode first aired on February 7th, 1964, just one week after The Alfred Hitchcock Hour aired its similarly-titled episode "Night Caller", also about a woman receiving mysterious anonymous phone calls at night.
Elva Keene's telephone is a Western Electric Model 202, which was in production when she had her life-altering accident in 1932. Although the phone itself appears to ring, the ringer was housed in a separate "subscriber set."