Alright, but how does a House Elf teleport to someone they have no real way of knowing where they are? No one knew where Harry Potter lived, yet Dobby was able to teleport into his home in order to try and warn him off, without the knowledge of his masters. This is somewhat separate, but ties into how a house elf's magic may or may not work.
It's never actually been shown, but I've always believed that invoking the house elf's name ties into their inherent magic, and thus grabs their attention to know they are needed when called, while also letting them know where whoever is calling them is located-- not that they're just somehow physically hearing someone call to them even in large manors or a castle like Hogwarts. This ties in well to things of a magical and fey nature: names have power, and also lends some credence into why people prefer avoiding saying the name 'Voldemort'.
Also, would not their duties expand into the grounds of Hogwarts itself? Cleaning the Quidditch stadium after matches, maintaining the various lawns, possibly working the greenhouses and such? Hogwarts isn't just the Castle, it's the grounds surrounding it as well. As such, I really don't see a reason we can't call on a House Elf just because we're outside of the main castle other than the fact that it makes some things trivial to deal with. We should be able to take advantage of the lack of common sense most wizards and witches display.
