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Windows Phone, like all the others, offers a voice control system but without some of the finesse or detailed features of other systems.
For example, asking it to text a contact in the contacts list returned the correct result without a problem, even prompting for the dictation of the rest of the message and confirmation of whether I wanted to send it. Google Now, by contrast, simply pops up the composition window. However, actually trying to use the system to dictate a message was incredibly frustrating.
Overall, the Windows Phone voice control feels a little less 'intelligent' than some of the others – asking it questions that you might ask any of the other systems usually results in a web search. For example, ask it to turn the Wi-Fi on or off and it can't help you, but it's bizarrely accurate if you use it to open apps on the device.
Think of the Windows Phone voice control system as less of a virtual assistant and more of a voice search tool – nearly everything you do will result in a Bing search, anyway.
Update: Some comments below pointed out the power of Windows Phone's app control system, so I went back for another look. Sure enough, while Windows Phone 8 did pose some frustrating problems in areas where others excelled it does offer features that the others do not.
For example, saying "open BBC News most read" resulted in the BBC News app opening and then the handset read aloud the headlines of each of the ten most read articles. Other apps also support this kind of action, with Audible allowing you to go directly to your library or specific audiobook by saying "open my Audible library", for example.
However, this functionality seems to be app specific and similar tests on apps such as Epicurious and Amazon could do no more than simply open the app.