So, trying to go for broke, Microsoft decided to publicly release the source code. The day of the public release is not yet decided: the code needs to be cleaned and organized first, so it will happen probably after July. Redmond hopes to attract, in particular, people who are disappointed by the NSA scandals.
Microsoft declared that all the source code of Windows Phone will be released, including the NT kernel.
Obiously, the NT kernel used for Windows Phone is not exactly the same that runs in personal computer's Windows editions.
The Phone NT kernel is called, by codename, "April fool".
By the way, if you are really looking for a "open source Windows" you may choose ReactOS. And if you want free software (not only open source), you should think about trying GNU/Linux distros like Debian (or Ubuntu, but it's not properly free software "out of the box" these days).