From my brief internet research, I uncovered some nice stories that helped me to understand why the signs were put in place. This discussion about Tiree (an island off Scotland) and some memories of Ballinasloe in Ireland by Declan Burke help to explain:

“Angus MacKechnie of Crossapol was recorded talking to Maggie Campbell of Kilmoluaig in October 2005 about his schooldays. He was strapped at school, deservedly so according to himself, for breaking the ceramic insulators on the telegraph poles on the road to Heylipol School.

This was a popular sport amongst schoolchildren, in Ruaig as well as Heylipol, and required accuracy in throwing stones. A boy’s reputation depended upon success, especially if the girls, who were guilty of the same activity, scored a direct hit.”

And:

 

“The Red Bridge had the salubrious effect with the railway embankment providing a wind-break so we could sunbathe. I always thought the suntan obtained at northern latitudes lasted a lot longer and looked better than that acquired in southerly climes. You could even get sunburn there.

Sometimes.

Amazingly, the Railway people were very tolerant of our use of their property, and sadly there was always some bastard who would abuse that trust. The ceramic insulators on the telephone poles were a very tempting target and these delinquents would destroy them with stone-throwing as casually as they would flick a cigarette butt away.

Then, ‘the man’ would shut off our access for a while, and who could blame him?”

 

Connected Earth gives some more explanation:

“These notices were screwed into telegraph poles to warn off potential vandals. The porcelain insulators holding the wires at the top of the poles were fragile and needed rapid and costly replacement. In areas where they were particularly vulnerable they could be replaced with more robust rubber ones.”

So, an interesting story of how telecoms networks used to be attacked by kids, even if it was just for target practice. Connected Earth states that the signs were in use from the 1910s through until the 1950s. Leave a comment if you know anything more about the story behind these signs!