Windows Phone Edition 1.0

Windows Phone Edition 1.0 was the first operating system for phones developed by Microsoft and Nokia. It was for a few phones Nokia developed.

Features
By default, you can call, text, and add to your contacts list a phone number. It also has a basic notes app. You can install custom apps on it by connecting your phone to your computer using a serial cable, and transfer over a PHE file to it using PHETRANS.exe in MS-DOS. App developers using the Windows Phone SDK can use its built-in debugger if they want to debug their apps.

April 1st, 1985 6:00 AM build
This is the only development build leaked, and the build that was released alongside the Windows Phone SDK, it's almost identical to the final phone version except it runs on a PC and debugging features.