[Johnythree]

This not strictly true: The original pairs were paper insulated, but they were enclosed in a lead sheath which prevented water entry. Plus the cable was pressurised with dry nitrogen to keep out any water. Plus the flow of Nitrogen was monitored so that any pinholes could be detected before they caused a problem. Of course when Telecom was privatised all these quality control issues went out the window.

But there's a further point, the problem with moisture in the lines is the corrosion caused by electrolysis, which in turn is caused by the 50V DC on the lines. So even with modern plastic insulation, the copper would be corroded away by any electrolysis.

One last point: Rubber was almost never used as an insulator on phone lines.