The other effect is skin effect - because of self-induction, higher frequencies tend to travel at the outer edge of the cable, so are attenuated more simply because they are effectively going down a thinner cable. But for typical telephone cable, the effective resistance increases by only 10% between 10kHz and 100kHz (over 10m, increases from ~1.7ohm to ~1.9ohm). so not noticeable to human hearing: https://en.wikipedia.org/wiki/Skin_effect