Benolam: If anybody wants to get really cheap mobile calls, then I would suggest trying the GlobalTel (US) Callback Service.
Their leg rates are :
NZ landlines (64) US 0.016
NZ mobiles (642) US 0.025
So a mobile to mobile call in NZ would only be US 5c/min, and between a mobile and landline only US 4c/min.
This is significant NZ mobile rates are high compared to other countries. Vodafone and Telecom prepaid is NZ 89c/min, 2degrees is better at NZ 44c/min (22c with monthly top-up to some numbers).
So what I would do is buy a $2 2degrees SIM from DSE or some other store (no longer available from 2degrees website) which includes $2 credit, and use the callback service from GlobalTel to make calls.
Cheapest option I can think of for casual mobile users.
You do you need to sign up for an account with GlobalTel and provide CC information and they bill you monthly, but I have been using them several months and they are a good crowd.
I'm not sure where you're getting those rates from because their website is telling me it's US 61c per minute to call from a NZ mobile to a NZ mobile!
There is certainly no way a telco or transit provider could be interconnecting with a NZ mobile operator and paying US 2.5c for termination!