I have heard them sing, but I was never quite sure if that was caused by electrical impulses or a simple resonance with a breeze. In any event, the sounds I heard were steady ones, akin to a low steady hum, not changing with any signal modulation ostensibly being carried by the wires.
That led me to conclude the sounds I heard were wind generated, although many times I felt the wind was insufficient to create a resonance. Moreover, the effect was not universally noticed on all pairs of poles. Sometimes, the wires would sing, but the wires on either side were silent.
That is the conclusion I am coming to. I did not know for sure, but I was unaware of any communication interruptions in the period 1910-1940, that from my reading of history, including the history of the telegraph. I therefore assumed electrical engineers had had to have done something to protect their networks while still allowing communications to be transmitted and received despite the detrimental effects of EM storms.
I have subsequently discovered that was not the case. There are numerous sources documenting the complete shutdown of these networks. Operators at the time just lived with it, learned to work with it, or went fishing until normalcy was restored. There may have been some shielding of some wires, but it was more the exception than the rule.
Back when? By 1930, I would presume scientists and engineers knew all they needed to know about the cause and effect of these EM storms.