Child line is not blocked in the default o2 filter. Its blocked in the opt-in filter, as is most of the net (it's a whitelist). This is clear if you use the o2 URL checker yourself.

While not agreeing with the filters, it seems that there's a large and vocal contingent who are being hysterical and getting the facts wrong. They are not helping.

Totally depends on the ISP.

Any mobile phone bought on O2 for instance in the last 10 or so years, had filtering enabled by default.

You just call them up and ask them to disable it. It's not a big deal.

I don't buy the ridiculous notion that the only means a child has to report child abuse is via the internet...

Some people will be embarrassed to make that phone call. Some people won't be able to do it because they don't have a credit card (if they're on PAYG, for example) and will have to go into a store, which is inconvenient, time consuming, and potentially embarrassing. For others, in the case of a wireline internet connection, the account might not be in their name and they will be unable to do it. I've had situations where the internet connection wasn't under my name but my housemate's name, and, of course, I couldn't have just called up or clicky clickied and disabled the censorship. I'm not someone that embarrasses easily but there are those that might not want to have that conversation with their housemate. What if those housemates decided that they didn't want porn going over the household's internet connection, or were too embarrassed themselves?

The next few generations are going to grow up with the idea that nanny filters and censorship is fine, or that porn is something shameful that you have to ask permission from a CSR to see. Then there's the sophisticated central apparatus in place for the government to trivially block any sites that they deem unsuitable, content they might deem exempt from their opt-out procedures.

Just because O2 and other UK mobile operators have been censoring for a while doesn't make it okay. Now you're arguing that censorship-by-default is fine because it's been going on in some capacity for a while already! See how this works?

Have you ever been abused? Children in abusive situations are scared; the internet provides some mild form of anonymous comfort, and is most likely where a child would report abuse. That or to their school, but often they feel that the school is "in on it" and the internet is safer because they are "an outside authority."

Source: I know a few adults who have been abused in the past. Anecdotal.

I didn't make the claim that it was "the only means," though I don't necessarily think such a scenario is ridiculous.

I do wonder why anyone would want to make it more difficult to report child abuse, though.