"Hello-Central" is a reference to a nineteenth-century telephone operator. Sandy chooses to give this somewhat bizarre name to her daughter because she hears her husband, Hank, use the phrase in his dreams as he still lives in the present when he sleeps. Note what the narrator tells us about how his daughter came to bear her name in Chapter 41:
In my dreams, along at first, I still wandered thirteen centuries away, and my unsatisfied spirit went calling and harking all up and down the unreplying vacancies of a vanished world. Many a time Sandy heard that imploring cry come from my lips in my sleep. With a grand magnanimity she saddled that cry of mine upon our child, conceiving it to be the name of some lost darling of mine.
Thus Hank, in his desire to "connect" with his own world, says the phrase "Hello-Central" in his sleep, representing the fact that in his dreams, at least, he still "wandered thirteen centuries away" and wasn't entirely happy or at home in Arthurian England. Sandy, hearing this phrase repeated so often, assuming that it was a person from Hank's own world who was very important to him.