Harry and Sirius Black use a two-way mirror to communicate with each other. One person holds the mirror and says the name of the other person, after which the two of them can see each other and communicate. Hello, free video chat on any smartphone platform!
Smartphones will undoubtedly be more useful, since the same “mirror” connects to any other “mirror” in the world. But there’s one catch: you need a data plan or network coverage of some sort, which the two-way mirror isn’t dependent on.