Line does have a voice call service. I have not used it yet so I cannot say how good it is.
But if you use that or another free voice call service, I suppose that if you always ensure that when you place a Line (or Skype) call you have wi-fi access and only use the net with wi-fi access, you could go with a minimal plan and save quite a bit since you are not going through your provider's towers. You do receive calls for free yes?
I could be wrong about all that. Or it might take some tweaking to accomplish. Still pondering....