IM+ Talk uses your data plan to send and receive text messages.
Skype-to-Skype calls in VoIP mode also use data plan.
In callback mode, small amount of data is used to place a call and then the call is forwarded to your callback number as the usual call. You will be charged for Skype call to the number you want to call, for Skype call to your callback number and (by your carrier) for incoming call to your callback number while in callback mode.
Shape Services Support Team