Microsoft Corp on Wednesday formally announced it has developed a voice activated phone assistant feature called Cortana, a direct rival to Apple Incs Siri. The feature has been rumored for some months and a test version was demonstrated by Joe Belfiore, a Windows Phone executive, at Microsofts annual developer conference in San Francisco. Belfiore announced that the latest version of Microsofts smartphone software, called Windows Phone 8.1, will be rolled out to consumers as a downloadable upgrade in the next few months, and new phones running the software will be in stores by late April or early May. (Reporting by Bill Rigby.