Viber, a popular VOIP application for IPhone has finally been released for Android.
A quick run down. The app works on push technology and essentially sits idle(like whatsapp), when a call is requested the app opens and connects the call. The idle data usage of Skype was one of the reasons I didn't leave it running on my phone.
According to their website a call will consume approximately 256k per minute so even taking into account our ridiculous R2/MB OOB rate here it works out to 50c per minute.
Another advantage it has over Skype is that is scans your contacts and automatically adds anybody who uses Viber to your list so you don't have to manually add people into your Viber contact list.
Give it a shot it is free.