Viber free phone call app to hit Windows Phone on Monday? Update: It's delayed

All week, the boss of Windows Phone Joel Belfiore has been teasing new apps about to hit the WP Store.

He teased the major update to Skype...then The Dark Knight Rising game...then Pandora music app.

Now, he's teasing one more...on Monday:

So what can we expect on Monday?

Instagram? Probably not.

Our money is on the long-awaited update to the popular messaging app Viber adding free phone calling. It would mean anyone using the Viber app on any smartphone on Windows Phone, Android, iOS and BlackBerry will be able to make free calls using WiFi or mobile data. And that would make it a real rival to Skype.

At the moment the Windows Phone Viber app is restricted to text.

Of course, perhaps...just maybe...it could be Instagram.

Update: Belfiore now says whatever-it-is has been delayed.