Truphone, probably the most famous on the mobile VoIP service works on Nokia E-and N-series devices, BlackBerry phones, iPhone, and IPod Touch, and Android based T-Mobile G1. I have tested it on the first generation iPhone, using the phone in a Wi-Fi connection. (You can use the cellular data connection on some phones.)
After downloading a free Truphone with iTunes, I have been running for several minutes. In the iPhone version saves iPhone look - if not for Truphone logo on top, you may not even realize that the iPhone Dialer does not own. Calls to other Truphone users are free, as well as the service allows you to access your account Skype (good for iPhone users waiting for a client Skype), as well as providing access to basic services for instant messaging.
In my test calls, voice quality varied greatly. Some calls sounded hollow, while others were loud and distracting background hiss. But it is very convenient service, and its sound quality was much better than Skype for mobile devices.
Truphone has two pricing options: TruStandard, which has no monthly fees and rates, which start with the 5.1 cents per minute, or TruSaver, which costs $ 4 a month, and rates that start at 2.1 cents per minute.