How do you feel about living in a society where the bare minimum of "this is legal right now" confers a moral imperative to provide someone with electricity if you're a power company, or with a telephone number if you're a telephone company?
What do you mean by harassment happening over the telephone? It could mean that someone calls someone else to harass them. Of course the victim could just block the harassers number in this case, but maybe the telephone company is justified in cutting off the harasser anyway.
But to stick with the KiwiFarms analogy: What if people talk to each other on the phone, ridicule others, share embarrassing information about others, and then some of them go on (allegedly) to harass the targets?
They might be able to deduce some of this (and some of it they can't even deduce legally), but it's nothing like an online forum in which the administrators and moderators condone the behavior to (what they see as) reasonable limits just shy of breaking laws against harassment.
Furthermore, if we likened web services to phone services, you'd find we're using a service which doesn't allow half of what KF allows to be posted in accordance with the HN guidelines. And I'm fine with that sort of moderation.
Most phone companies are set up with the implicit assumption that they are content neutral within the bounds of the law, though some do provide further restrictions. KF was not set up as such - this 'ridicule' and 'embarrassing information' is the entire point of the website, and is much closer as a proximal cause to the harassment. It aided that ridicule and connecting people of like minds with bad intentions with the knowledge that this kind of discussion is explicitly banned on Reddit, HN, and to some extent 4chan.
If there were a phone company set up with its advertising and moderation similar to KF's, I'd say the phone company is just as culpable as KF.