Nokia, the world's largest mobile phone maker, said Friday it was joining forces with U.S. giant Microsoft in a major corporate strategy shake-up that left investors disappointed.

The Finnish phone maker said Microsoft's Windows Phone would now serve as its primary Smartphone platform in a "broad strategic partnership" destined to regain market share from competitors Apple and Google.

Worldwide mobile sales increased 32 percent last year and Smartphone shipments surpassed those of personal computers for the first time in the fourth quarter, according to tracking firms.

Technology research firm Gartner said Wednesday that mobile device sales rose 31.8 percent last year to 1.6 billion units over the previous year.

Nokia, the world's leading mobile phone maker, said Monday that its new E7 business Smartphone, whose release had been delayed from the end of 2010, will be on sale in "select markets" this week.

The E7 "will begin arriving in stores in select markets this week, with broader availability building up quickly in several markets," the company said in a statement.