7 of 7Image
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.
Voice control has been around for a good while now, so it's frustrating that it still isn't perfect as that's what it really needs to be if it wants to be a viable way to interact with your phone in a dependable way. You wouldn't be very happy if half the time you pressed a button it did what you wanted and the other half it did nothing, or worse, something you actually didn't want — and sadly that's a big part of the experience with voice control.
Using the software indoors (where these were tested) improves the chances of it doing what you want, and out of the bunch Google Now was the most useful and best at recognising what was being said, that's not to say that Siri has nothing to offer — clearly its partnership with Wolfram Alpha is one of its strongest points, and a factor that other systems would do well to provide in some way.
As for Windows Phone's voice control in my experience it's just not good enough to be useful or reliable — although it is a handy way to quickly open apps.
BlackBerry's system sits somewhere in the middle of the pack, it has hardware functionality like being able to switch Bluetooth on or off, but its actual recognition is a little hit-and-miss at times: still, it fared better than Windows Phone 8.
The question isn't really which voice control system is better, right now it's more about which has the biggest limitations.