jjnz1: I agree with TomS, i think "the more people who sign up, the more you will save" refers to the more people on 2Degrees, the more "onnet" calls you will make.
I think 2D's onnet calls will be almost free.
If all your friends sign up, then you could call them anytime of the day for little price.
I've been thinking about this as well, and I remember Telecom/Vodafone was trying to stop them issuing local are code numbers (e.g. 09, 04 etc) for their mobiles and then having roaming within huge zones unlike the LocalZone Vodafone offers.
If they had to back down on this, I can imagine the free SIM/$5 credit will be credit to say 022 numbers only with what 3-5c/minute calling - enough for a long time.
They have a marketing budget, and a issue of quickly ramping up customers, or trying to get customers on-net - why not make on-net nearly free and carpet bomb SIM cards.
If I could call all my friends for nearly free on their mobiles, if I used a 022 SIM which was free itself, it's only lazyness stopping me...