It could be the most expensive call Microsoft Corp. has ever made.

In acquiring Internet phone service Skype for $8.5 billion, the technology giant is seeking new ways to make money as its core computer software business faces a growing threat from a new generation of powerful mobile devices.

But some analysts believe the Skype deal, Microsoft’s largest ever, could become a multibillion-dollar dud, as it once was for EBay Inc. The online auction site acquired Skype for $3.1 billion in 2005 but then sold most of its stake in the phone service after failing to wring a profit from it.