Editor's note: this is the third and final part in a series of stories on the behind-the-scenes efforts by Microsoft to bring Windows Phone 7 to market.

REDMOND, Wash.--On a rather sunny August day deep within one of the many nondescript buildings that dot Microsoft's campus, a robot taps away at a prototype Windows Phone 7 to double-check that the screen is accurately reading touch input. Another robot, affectionately dubbed Wally, tests the accelerometer and other sensors built into the new phones. A building over, thousands of phones are going through a variety of stress tests, including both automated and hand-performed tasks.

That's a major shift for Microsoft, which in the past focused on making sure its software was bug-free and left much of the testing of final phones to the partners that make the actual Windows Phone devices.

"We really felt like our product quality ensuring system infrastructure wasn't good enough," said Darren Laybourn, general manager of test for Microsoft's phone unit. Laybourn and his team allowed CNET an exclusive view inside the testing labs as the company was putting the final touches on its software and helping its phone maker partners gain approval from various cell phone carriers. "We spent over $15 million on hardware...we've added hundreds of senior people to the team, we've completely revamped our processes."

Microsoft used its beefed-up testing methods as part of its effort to convince carriers that it was serious about getting back in the phone game.

"That was kind of my story," Laybourn said, showing a slide deck he used in pitches to AT&T and other cell service providers. "Here's why we suck less, basically."

Indeed, Windows Phone 7, which went on sale in the U.S. on Monday, is arguably one of the most heavily tested products to come out of Microsoft, save perhaps a full release of Windows or Office.