Trace, I can only speak from personal experience. My office is located about 9 miles from our serving CO and there is one underground "hut" along the cable route. Chances are, there is a doubler there. We lost our circuit about a year ago and I was told that it was due to a power supply failure in this hut. This implies that there must be some type of electronic device associated with our circuit in the rack that lost power.
From what I've been told by Verizon technicians, our circuit is on a copper pair the whole way. I know of at least six of these circuits that are working in our office park cul-de-sac, all riding on the same 900 pair distribution cable down our street with no issues whatsoever.
The feed to this general area is comprised of two aerial 2,700 pair 26 gauge cables and one 2,400 pair, 26 gauge cable. There are certainly visible repeater cans and load coils along the route, but they are obviously for unrelated pairs to mine.
I've tried to beat up our circuit a couple of times by shorting or grounding the pair and it always comes right back up. Our distribution cable is 1970's era PIC, so it's not in the best of shape. With that being said, I'd say that it's safe to assume that these circuits are fairly bulletproof. :rolleyes:
