If TX and RX are not on same phase, they will still communicate but there is a loss and that would limit the range. This might be compensated with higher gain.
For example, in a utility metering application, a Server (DCU) is generally connected to all 3 phases via 3 phase coupling (i.e. using 3 Line drivers). Each meter is connected to one phase and meters are talking with the server, not the meters on the other phase.
In smaller scale cases, like in-home, we have seen single phase server (gateway) connected between L1-L2 (or even L1-N), and meter module between L1-N or L2-N. They can still communicate because the range is shorter, and you can boost the gain.