As recently announced, the MyNetFone app is available from Apple's App Store [link to iTunes Store] for no charge and using it doesn't cost anything - it's an extra service that's made available to all iPhone and iPod touch owners running iOS 2.2 or above. Installation follows the same process as any other application.
Once installed, you'll need to provide the app with your MyNetFone username and password. This is not the same username and password that is used to access your account. This is the SIP username and password that was provided in the Service Confirmation email that was sent when the MyNetFone account was first created.
With everything in place, we made and received test calls over WiFi from open hotspots and over 3G. With iOS able to keep apps running in the background, once MyNetFone has been launched it's able to receive VoIP calls. This fully integrates with the iOS Address Book and call log so you can easily track calls and don't have to maintain a separate contact list.
Call quality was highly dependent on our Internet connection. With an iPhone 4 running the Release Candidate version of iOS 4.2 we couldn't tell the difference between a VoIP call and regular mobile call. Over 3G though, where we had a solid signal calls were fine but, like any call over the cellular network, the call quality wavered between good and useless.
Data usage for making and receiving calls is difficult to track but MyNetFone's FAQ says that the default G.729 CODEC uses about 1 MB for every 10 minutes of conversation. This can get as high as 1MB per 1 minute if the G.711A/u CODEC is used. However, we couldn't find how to change this within the MyNetFone app.
As well as supporting iOS, MyNetFone has released Android and Symbian versions of their app.