Rambling on about Geek stuff 90 months ago.
You can either join the free plan or a paid plan ($15 - $30 per month) depending on how many outgoing minutes you want. You are provided with 028 number (and a regional one if you choose a paid plan).
Outgoing calls cost 5c for NZ national calls, 25c for NZ mobile, and the normal cheap rates for international calls.
Incoming calls are charged at mobile rates to your 028 or normal national/local rate for a regional number. and it rings on your software phone or SIP hardware phone if you have one. You can have incoming calls simultaneously ring (or redirect to) any NZ number, but you have to pay your outgoing call rate for it.
In summary, nothing too revolutionary going on, but it's always good to have some more competition in the New Zealand telecommunications market and I look forwards to see whether 2talk picks up much steam.