One possibility is the usage of powerlan. Several companies like TP-Link, AVM, Devolo, etc. offer so called powerlan adapters, that use the grid of the flat or the house to establish a network for data transmission.
Concerning the hardware you need:
To establish the network you hava to:
That's it. The connection over the grid is of course secured. It depens on the producer how many bits they are using for the key.
Concerning the phases in your grid: If you need to send the signal from one phase to another, don't worry, many producers offer adapters that can handle this situation (they are just a little bit more expensive).