Latest known Windows phone builds being tested internally at Microsoft: WP8.1 Update 2 & Windows 10 for phones

According to Nawzil (Ex-Microsoftie) Microsoft is working on the next version of Windows Phone.At present Microsoft is working on 3 Windows Phone builds which is not a surprise to us :

• WP8.1 Update 1 : 8.10.14200
• Next version Update 2 : 8.10.15XXX
• The next big version of Windows Phone i.e. Windows 10 for phones : 8.15.12434

Windows Phone 8.1 update 1 will have another minor update.
Windows Phone 8.1 Update 2 (GDR 2) is rumoured to bring features such as:
Sorting of Settings finally comes to Windows Phone and it will be alphabetically with an option to reach to settings you want just like the apps list.Mobile data toggle will be finally present in Action center as one of the options Cortana will go to more regions but still will be in Alpha.

Windows 10 for phones is a mystery yet,the OS is expected to bring API unification with Windows 10 Metro which will deliver side by side applications for phablets.There will we also a slight UI modifications to make it more accurate with Windows 10 Metro such as app groups.Windows Phone for tablets and finally the full Office Touch (Gemini) for Windows Phone.Windows 10 for phones may be shown off in Q1 2015.

P.S. : The above image is cropped and then posted in the blog which was announced in Windows 10 release.