Ham Radios would be good. My brother built one and I know that you have to pass different tests for the licenses. The first license you could get you couldn't talk you had to use Morse Code. I remember this very well because I helped him learn Morse Code. The next license was when you could talk. Don't remember the details this was back in the 1960's and I'm younger than him. I just know that you had to take tests to get your license which gave you your call sign. So Ham Radios would be best because you can use either Morse Code or talking and they don't require wires just that each radio be attached to an antenna.
If you want an idea about it I believe the movie is called Frequency. I haven't seen it but the plot of the story is that a young man on a Ham radio some how talks back in time to his father before he died.
