You don't incur any data if you're using Callback (Autodialler)  or Callback (Callthrough) as calling method. You only consume data if you're calling method is set to Callback (Internet) or Callback (SMS). These are also the only available calling methods when overseas.

Hoiio callback only uses data connection to initialise the call. Your phone can connect to either GPRS/WiFi, and the size of data connection for each call is around 4 KB for call back, and around 5 KB for sending an SMS