On January 10, a "Phone Insider" app was found in the Store that was later confirmed on Twitter by Gabe Aul to be an important topic at the January 21 Windows 10 event. This lined up with earlier rumors that Microsoft will reveal more about the phone version of Windows 10, and sparked a great deal of speculation about the naming and design of the new OS.
Windows Central was first to report that a new app called "Windows Previewer Beta" has been spotted in the Store. The app is reportedly an internal version of "Phone Insider" and as such it is currently unavailable for download, but Windows Phone developer and XDA forum moderator Jeremy Sinclair has done some digging and even provided a few screenshots.
As with "Phone Insider", the new app was designed to give users early access to Windows updates and provide feedback to Microsoft. Jeremy Sinclair, who has analyzed the app in more detail, shared a few details about what's new in "Windows Previewer Beta":
Basically, this app enables a few new Authentication methods. First it tries to auth with the built-in Microsoft Account on your device. If it fails for any reason such as not having access to any Windows Phone flights for the Insider program, it leads you to the AlternateLogon page. This page allows for you to manually login with a Microsoft Account or to login with an internal Microsoft account.
Still not useful for us who haven't been deployed access to all the joy, but this shows as confirmation that the next update to the Phone Insider app will involve a rename and allow for us to login to it once it goes live.
Again, nothing works at the moment, since this is a private app, but it does suggest that Microsoft is close to revealing the next preview version of Windows for phones for all Insiders. It also fuels speculation about the naming of the operating system, with possible options being "Windows 10 Mobile" and "Windows 10 for Phones".
Source: Windows Phone Store via Windows Central