At the backbone level, perhaps, but for historical reasons cable and phone companies were separate, and thus we have both Comcast and Verizon providing last-mile communication services. I already have both phone and cable lines to my apartment, so it's more of a natural duopoly there, and even duopolistic markets should have nonzero competition.
It was just as much a natural monopoly historically as it is now. The last mile cost you talk about is the barrier to entry I mentioned. Again, it's not a very good example because (historically or not) it's a market that lends to less competition by default.
At the backbone level, perhaps, but for historical reasons cable and phone companies were separate, and thus we have both Comcast and Verizon providing last-mile communication services. I already have both phone and cable lines to my apartment, so it's more of a natural duopoly there, and even duopolistic markets should have nonzero competition.