Microsoft is gearing up to release Windows Phone 8.1 in April, and one major step towards this goal is giving developers access to the relevant pieces of code ahead of time, to ensure their applications are ready for when the OS is available to the public.

Microsoft has been sending out invites giving select Windows Phone developers access to Windows Phone 8.1 SDKs and operating system builds. The updated OS is set to be detailed in full at Microsoft's BUILD 2014 developer conference in April, after which it will be available to all through the Windows Phone Preview for Developers program.

Ahead of BUILD, Microsoft is reportedly planning on sharing many Windows Phone 8.1 details behind closed doors. A "confidential" event for close partners is being held in London on February 28th, where business customers will be briefed on Windows Phone 8.1's technical details and roadmaps.

Also in the lead up to Windows Phone 8.1's release, Microsoft will make an appearance at Mobile World Conference towards the end of this month to discuss their plans for growing Windows Phone. Windows Phone 8.1 won't be specifically shown off at this event, as Microsoft has yet to publicly acknowledge the existence of the operating system update.

New features to expect in Windows Phone 8.1 include a notification/action center, which was pictured yesterday, as well as a voice-based personal assistant (similar to Siri) codenamed 'Cortana', separate ringtone and media volume controls, and screen mirroring settings.