Viber (website) a VOIP program similar to Skype, is quite popular on the iPhone for its no-frills ability to call any other Viber client in the world for free. The app though is in someways better than Skype since you don't need to sign up or register--you just download the app and use it. The app is quite pouplar on the iPhone and there are Android and BlackBerry versions coming soon.
Well, unfortunately for the same reason as Skype, they too are saying no client from them until Microsoft allows deeper access. WinMobile.SE interviewed Talmon Marco, head of Viber Media and asked him about WP7 directly. He cites three main reasons why, despite them wanting to do one, they can't make a Windows Phone app: (1) no sockets support, (2) no native code access (so can't port audio engine) and (3) no multitasking.
Once again, no surprises here--if Skype can't do it, Viber can't. Still, it seems both parties are interested, so it's just a matter of Microsoft allowing access.
Source: WinMobile.SE; Thanks, Toni, for the heads up