Microsoft Phone Insider app hints at a preview version of Windows 10 for phones
Microsoft already has a Preview for Developers app that requires users to have an active Windows Phone Dev Center account, an active App Studio account or use a registered developer device. This app allows Windows Phone users to install pre-release versions of Microsoft's mobile OS before the carriers receive them.
Using the Preview for Developers app could void your warranty, something that doesn't seem to be the case with the Microsoft Phone Insider app. Additionally, registering with the Windows Insider program means being able to send Microsoft feedback about any pre-release OS you install. While Microsoft hasn't said that a preview version of Windows 10 for phones is going to be released, the fact that this app might soon be available to install is a strong sign that such a preview will be offered.
source: PhoneInsider (Windows) via WindowsCentral