This only works if there is power and if you all are on the same power transformer.

We have used a set of FM intercoms that you just plug in and it uses wiring to send the signal. Some years ago, we had a set of them between our home and our daughter's home, which was across the street - but we were all on the same transformer. They work fine between the house and the garage or barn or any other building on your property and the same power system.