A Nokia Lumia 920 smartphone was used to unveil Microsoft's Windows Phone 8 software at an event in October 2012. (Tony Avelar/Bloomberg)

Microsoft and Nokia have completed their deal to sell the Finnish firm's devices business to the tech giant in Redmond, Wash. The $7.2 billion bid, announced in September, has made it through all of the regulatory hurdles, transforming the old Nokia business into Microsoft Mobile.

Now the hard work starts.

Microsoft is well aware that it has a lot of work to do not only to catch up in mobile and but also to figure out what comes after mobile. In an earnings call on Thursday, Microsoft chief executive Satya Nadella -- who's just a few months into the job -- said that the firm is spurred on by the knowledge that it has to face facts.

"What you can expect of Microsoft is courage in the face of reality," Nadella said. "We will approach our future with a challenger mind set." (As CNET notes, Nadella has apparently read his Nietzsche.)

Microsoft doesn't have a particularly impressive track record on mobile, having come late to the smartphone and tablet markets. Nokia -- er, Microsoft Mobile -- has created a strong, high-quality set of products with the Lumia line and still has strength in emerging markets such as Latin America and Asia. That's a good start for Microsoft as it looks for growth in the market, but what the company really needs to do is look ahead.

Because, while Windows Phone is the fastest-growing smartphone operating system right now, analysis firm IDC still expects those models to account for only 3.9 percent of shipments in 2014. By contrast, Android is forecast to take a 78.9 percent piece of the overall smartphone market; Apple is expected to take 14.9 percent.

Nadella's move to make Microsoft products more open-source -- such as offering Windows for free to "Internet of things" developers -- is a good first step. No, Microsoft can't just give up on mobile. But it also has to buck its own failures and work hard on leapfrogging the rest of industry so that it doesn't repeat history.