Yip. Check out Virgin, Vodafone and 3 in Australia who all offer unlimited "free" on net calls with most of their new plans.
Vodafone & 3 Aus is not a great comparison, since they are a JV.
And it's not really "free" on Virgin either; you need to pay at least $19 each month to get it, and you must have at least 1c balance. In other words, it isn't free, it's $19/mth. And it's also worth noting that mobile calling rates on mere mortal plans were already under 30c/min when I left Aus in 2002. They've had real competition for a lot longer than NZ.
I would pay $30-50/mo for unlimited on-net/landlines calls. Means no more landline!
Would you have issues with the Commerce Commission if they prevented the launch of such a product?