Skype will launch its VoIP application for Apple's iPhone on Tuesday, and will debut a similar application for some of Research In Motion's BlackBerry devices in May. 

The Skype for iPhone application will allow users to make calls to other Skype users for free if they are using a WiFi hotspot. Skype users can also call landline numbers for a fee, usually for around 2.1 cents per minute. The application will use numbers taken directly from the iPhone's address book, and will even display a photo of the contact (if they have uploaded one for Skype).

Skype, a unit of eBay, has made it known in the past that it planned on pushing its service more directly into the cell phone market, branching out from its traditional personal computer base. Skype currently offers its calling application to Windows Mobile smartphones and in January began offering a version of the software for use on the T-Mobile G1, the first phone in North America running on Google's Android platform.

Skype recently tussled with O2 and Orange when the two carriers declined to sell Nokia's Skype-enabled N97 handset.
 
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