It appears that Microsoft is internally testing Windows 10 on smartphones, according to an ex-Microsoft employee. Although there's not much information available about what to expect from Windows 10 on Phones, it is rumoured that it might bring side-by-side multitasking on tablets and full Office Touch.
Windows 10 Technical Preview has been out for couple of weeks now. When Microsoft first announced the Windows 10 back in September 2014, it announced that Windows 10 will be true 'One platform' for all devices, that is, the same operating system and features will be available on phones, tablets, laptops, desktops, etc. Does this mean that we might finally see a desktop like open environment in smartphones?
Microsoft will, essentially, converge the platform in a way that is the same at the system level on all devices, but with the UI optimized for the form factor. However, it needs to be seen how it manages to overcome redundancy in features. There might be few features that one might find useful on tablets but not on phone and vise versa.
Having said that, Microsoft isn't only testing Windows 10 on phones internally. It's also working on WP8.1 Update 1 and one for the next minor version Update 2. Windows Phone 8.1 Update 1 will bring Cortana to many handsets, while the Windows Phone 8.1 Update 2 is rumoured to bring features such as a data toggle in the action centre, sortable settings, support for new processors and Cortana coming in Alpha to more regions.