I see that 2degrees states in their T&Cs:
(e) You may not make 0900 calls, collect calls, special number calls, or any calls where charges are not immediately deducted from your Pay Now Account, or use your Mobile Phone for voice over internet protocol (“VOIP”) or message over internet protocol (“MOIP”) communications.
Although you'd want 3G data services to do VoIP, does this mean that they do not allow users to use SIP clients over VoIP from their mobiles? I have not see such conditions from other mobile operators, so it seems a strange condition.
Maybe ensuring all calls result in payment to 2degrees is the way they keep their rates lower than others.