The landline only work if you are standstill. As soon as you have open war, even just over 50m no-mans land, you were out of reach, you lose the connection and

The landline only work if you are standstill. As soon as you have open war, even just over 50m no-mans land, you were out of reach, you lose the connection and it takes a while to catch up. Specially as artillery shells destroyed wires. So pulling a phone over a few 100m of no-mans land was not that hard, these places would be shelled when the counter-attack happens. So yeah, in pure trench war-fare they had comms, but coordinating an ongoing attack is really what you can't do.