As part of the mammoth series of updates coming in Windows Phone 8.1, Microsoft will for the first time integrate Skype with its smart phone OS. And that version of Skype will be improved over the currently available app, with deeper OS integration and a number of new features.

New features in Skype for Windows Phone 8.1 include:

Cortana integration. Skype will integrate with the Cortana virtual personal assistant so you can start Skype calls with your voice. ("Skype, get Paul Thurrott on video" and so on.)

Phone app integration. With Skype now part of Windows Phone 8.1—just as it is part of Windows 8.1—deeper integration is possible. This includes Skype integration with the Phone app, so you can "upgrade" a regular phone call to a Skype call or video call. Skype says that you can now preview the feature in the current Store app, and promises further integration bits in the next few months.

Always online. Thanks to the aforementioned integration, Skype is configured to be online all the time by default. This lets you stay connected with others at all times via real time notifications and Skype tile updates.

People hub integration. Your Skype contacts are available in the People hub, so you can launch video calls directly from your contacts list.

Video instant messaging. This feature lets you leave video messages for contacts that are offline.

HD 720p video calling. Video calls can now be done in HD quality.

Improved user experience. Skype says that "navigation through the app using the beautiful modern user interface." I've not had a chance to check out this app yet, so it's not clear what's changing, but Joe B. highlighted a few screens during his Build talk.