Phone giant AT&T sees big opportunity in building its own applications.

The phone company Wednesday announced it’s buying privately held Plusmo for an undisclosed sum to help it build widgets and apps for mobile phones, PCs, and TV. Details of the transaction were not available.

Plusmo builds applications and widgets for mobile phones. But because it uses common Web development standards it should be easy for AT&T to use its technology to build applications across multiple operating systems. Once the acquisition is complete, Plusmo will become part of AT&T Interactive, AT&T said in its press release.

While most people don’t think of AT&T as an application developer, the company has developed some of its own mobile apps. For example, it has developed several apps for the iPhone App Store, including a YellowPages app. It has also created applications for mobile phones other than the iPhone.

The acquisition of Plusmo should help AT&T develop new applications more quickly and more cheaply. It could also allow for over the air updates for mobile devices. But AT&T is also planning to use the technology to develop applications for its broadband customers and TV subscribers.

Plusmo says it offers a service that delivers more than 20,000 mobile widgets to consumers. It has also developed numerous sports-oriented applications offered on several platforms. The company has relationships with handset manufacturers, publishers, and carriers, according to AT&T.