Microsoft's "Windows 10: The Next Chapter" event on January 21 is fast approaching. And Microsoft is priming the Windows Phone pump for the expected Windows 10 mobile preview that the Operating Systems Group team has been readying.

Windows 10 mobile is the version of Windows 10 that is expected to work on Windows Phones and smaller Windows tablets running ARM and most likely Intel processors, sources have been saying for a while.

Over the weekend, Microsoft made available in the Windows Phone Store a "Phone Insider" application. Currently, that application is restricted for use by Microsoft employees only, but many of us Microsoft watchers are expecting the company to use this application as an aid for discovering the upcoming Windows 10 mobile preview builds.

The description of the Phone Insider application, which requires the Windows Phone 8.1 operating system to run:

"The Phone Insider application provides registered Windows Insiders the ability to receive pre-release OS updates on their phone, directly from Microsoft. For more information about registering and becoming a Windows Insider visit http://insider.windows.com."

In December, Microsoft officials said more than 1.5 million people had registered as Windows Insiders, enabling them to test Windows 10.

Microsoft is expected to show off a first public test build of the Windows 10 mobile SKU on January 21, according to sources. I'm not sure if that test build will be available on January 21 or shortly thereafter, but I'm still hearing it might be at least a few days after that event.

There's also likely to be an updated preview of Windows 10 for desktop PCs on or around January 21. The most recent officially released preview version of Windows 10 desktop was in November (though there have been some unofficial leaks of more recent builds since then).

Microsoft's Gabe Aul, head of the Data & Fundamentals in the Operating Systems Group (and head of the Windows Insider program) has tweeted previously that Microsoft will reveal the date of availability for coming Windows 10 preview builds on January 21, but he has not said what those dates will be.

I'm expecting Microsoft to show off the new "Spartan" browser it is building as part of Windows 10, as well as a preview of the touch-first version of Office for Windows at the January 21 event.

Microsoft officials have not provided an agenda or any information on what the company will showcase on January 21. Officials have said to expect Microsoft to live stream the morning keynote, but have not yet provided details on where/when that keynote will be available.