While we've heard several times before that Microsoft is working on a voice assistant for Windows Phone, we haven't seen any proof of it. Details keep coming up about the capabilities of the alleged voice assistant, including the ability to unlock the device using voice controls or the always listening mode, but as I said before, it's all blind firing. According to a MSFTNerd, Cortana will make its first official appearance in April, and will launch as beta on Lumia devices for US developers.
The same leaker also mentioned the voice of Cortana will be played by Jen Taylor, the same celebrity who's best known for her voice as Cortana in various Halo games, Zoey in Left 4 Dead, and a bunch of other games. Coming over to its launch in April, it's a possibility as BUILD conference will take place in San Francisco where the Redmond giant is also expected to unveil the upcoming Windows Phone 8.1 operating system.
Interestingly, Microsoft's expected to expand the functionality of the voice assistant to its Bing application on iOS, followed by US Xbox and Windows users in 2015 along with the highly anticipated Threshold release. Other English-speaking countries have to wait a little longer as the availability is pegged for 2015-2016 for them.
The rumor gave us some interesting information, but I'd suggest readers to take it with a grain of salt. There's a high probability the Redmond giant will showcase the upcoming Windows Phone 8.1 at BUILD in April, and Cortana should be a part of it, but nothing is official until it comes from Microsoft itself.