I have also seen the phone line and insulators at several points along Elk Lake Creek. It's a fairly heavy-gauge wire and is in surprisingly good shape considering how old it probably is. I always wonder when I see it where it led to. Lookout towers? Ranger stations? Does your book address specifics like this?
Could you give us some details about the book you have (full title, author, publisher, date, etc.)? It might be hard to find a copy, but I'd like to give it a try. It sounds like fascinating reading.
Regarding the phone lines on Elk Lake Creek, I have seen wire on the ground on the trail about 1/2 mile past the second creek crossing (heading from the Collowash trailhead towards Battle Creek shelter). Also off the trail near the edge of the creek about 3/4 mile upstream from the Collowash bridge. The wire at this spot looks like it was dragged downhill to the creek by a rockslide. There's also an insulator but no wire high in a tree along the trail shortly after the first creek crossing. Good luck finding this one! I had hiked this trail at least a dozen times before I noticed it. You could probably spot more insulators up in the trees along the trail if you really look for them.
At the campsite on the Collowash there is some of this same telephone wire wrapped around and between the two trees where the storage cabinet is attached. This further supports the idea that this segment of trail was part of a major trail at one time.