“AT&T’s surprise decision Tuesday to reverse course and permit low-cost Internet calls to be made over its cellular network is good news for iPhone owners, but it leaves Apple with some explaining to do,” Philip Elmer-DeWitt reports for Fortune.

“Apple was quick to welcome AT&T’s change of heart. ‘We are very happy that AT&T is now supporting VoIP [Voice over Internet Protocol] applications,’ said Apple spokeswoman Natalie Kerris. She promised that the company would get updated versions of Internet-calling apps — such as those made by Vonage and eBay’s Skype — up on the iPhone App Store soon as possible,” Elmer-DeWitt reports.

“But Apple had nothing new to say about Google Voice, the high-profile telephone management application that Apple declined to approve last August — triggering a government inquiry that revealed that it was Apple, not AT&T, that blocked it,” Elmer-DeWitt reports. “So why is Apple OK with Skype and Vonage, but not with Google Voice?”

“The problem Apple had with Google Voice, according to the company’s response to the FCC inquiry, was that the app replaced the iPhone’s ‘core mobile telephone functionality and Apple user interface with its own user interface for telephone calls,’” Elmer-DeWitt reports. “The problem with that explanation… is that the same is true of Vonage and to a lesser extent Skype. Both replace the iPhone’s core mobile telephone functionality and user interface with their own.”

“In this context comes Google and Verizon’s announcement Tuesday that they are bringing Google’s Android operating system — and its Google Voice app — to Verizon’s networkt,” Elmer-DeWitt reports. “The pressure on Apple is mounting, although with an installed base of 30 million iPhones and 85,000 apps, it still has the upper hand.”

Full article here.