In Short

Windows Insider for phone (previously known as Phone Insider) is a new application released by the Microsoft Corporation to the public in early January 2015. The app is for Windows Phone 8.1 devices and was previously an internal application for carrier and OEM partners for testing new versions of the Windows Phone OS.

Windows Insider is assumed to be an extension of the Windows Insider program for Windows 10 Mobile.

Windows Insider for phone appears to be analogous to the Preview for Developers program, which lets users download the latest official versions of the Windows Phone OS. However, Windows Insider is thought to let consumers try pre-production versions of future iterations of the mobile OS, serving as proper beta testers. Since the Windows Insider app was released the Preview for Developers program was discontinued.

Users give feedback to Microsoft about the OS through an app to collect data, ratings, and user comments. That feedback goes back into improving the OS for a final release, which for Windows 10 is not expected to be completed until mid to late 2015.