In Harry Potter and the Prisoner of Azkaban Harry and Ron get lost on their way to Divination class and get help from a bumbling knight in a painting. After they find the way the knight offers to help them again sometime and Ron says, "Yeah, we'll call you." At least in America the expression "we'll call you" when you don't really want to see someone again comes from telephone culture. It doesn't seem like a phrase that non-muggle born magic folk would use.
Was this a perhaps a mistake akin to an anachronism on the part of Rowling? Or is there perhaps some non-telephone related phrase in England that I'm unfamiliar with? Something else?