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.

August 30th, 1984 build (Nokia 328 demo)
For some reason, the demo for Nokia's model 328 (A Pocket MS-DOS device with phone features.) uses a very early build of Windows Phone 1.0 as a runtime. While the build date of the customized build can be retrieved, its unknown what features the unmodified version of the build has at the time of its compilation due to how crippled the customized build is (Its only purpose seems to run the mentioned demo).

December 30th, 1984 build (Microsoft BBS; Developer Release 3)
The build date to this build was accidentally posted onto the Microsoft BBS around 1988, nothing was known about this build until a batch of floppy disks containing the build alongside an early SDK were dumped and shared by a former Microsoft employee by January 2021. Unfortunately portions of the early SDK were corrupted, however the build itself can still be retrieved, more investigation is due. (The Developer Release 3 name was taken from the labels of the floppy disks, witch all said "Windows Phone Developer Release 3".)

April 1st, 1985 6:00 AM build (Windows Phone SDK)
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.