“Skype has now released an official mobile client. The application is written in Java so it should run on a variety of phones including all S60 and UIQ phones. The application uses an Internet connection for status and contact information and for Skype chat (instant messaging). However while the application does allows you to make calls, it routes them through the PSTN (phone) network rather than over the Internet. This means you will be charged for both incoming and outgoing calls of any type. Read on for more.”
PS: voice calls are not over data!