Are Chinese mainland citizens, as has been reported, finding their telephone conversations cut off whenever they mention the word "protest?" While large-scale, real-time voice recognition is a technological possibility, it is at the edge of what is believed likely. It would certainly be revealing about the capabilities of the Chinese government if these anecdotes proved to be widespread.
Such a powerful capability is generally seen as something Chinese authorities would want to keep confidential, rather than reveal in simplistic blocking strategies such as single-word phone hang-ups. That said, it is entirely understandable that technology users in China would believe such behavior from their telephone system providers. Much of China's Internet blocking works in exactly this way: You use the wrong term on a search engine, and your connection to that search engine mysteriously shuts down for a few minutes.
In the last month, users of Google's Gmail service have reported random interruptions and slowdowns in service. Many of them complained to Google, assuming that the breakdown was on Google's end. In fact, Google and others report that the slowdown came from within China, and was deliberately targeted at the service. Christine Chen, a Google executive, told the IT news outlet IDG News Service: "There is nothing technically wrong on our side, so you will have to ask the [Chinese] government as it is clearly an issue on their end".
Those using commercial virtual private networks (VPNs) to bypass such censorship have also found their connections mysteriously fail. As Oiwan Lam at Global Voices Advocacy reports, the Great Firewall has, in some places, been expanded to block the Internet protocols that most VPNs use. Blocking at such a low level can look like a failure of client software, rather than a deliberate act of governmental interference.
Such blocks are far easier to implement than telephone voice recognition. They're also relatively easy to circumvent. But the constant cat and mouse game between those who are blocked and those who seek to bypass those blocks serves to remind everyone that the Chinese authorities are always watching.
Some governments are happy to present their censorship openly (even jocularly, as with Qatar's blocked page). But China mostly prefers to implement its censorship in the shadows. The self-censorship of Chinese social sites is cloaked in euphemistic language, as this slide-show of recent censorship messages collected by CPJ board member and New America Foundation senior fellow Rebecca MacKinnon shows. When the Tibetan affairs site TibetCul was closed earlier this week, the server owners refused to tell the operators of the site why it had been disabled, except that "higher authorities" had ordered the closure.
High-tech censorship is rarely perfect, but it does not have to be. The arbitrary and random nature of China's information blockade has the advantage for the Chinese authorities of making everyone paranoid of its true extent or who will be next. Does China have the technology to recognize and block the word "protest" however you say it? Probably not. But if believing that means that you are less likely to write about any protests, or talk to others about them, it is effective censorship nonetheless.