Going by that logic, You have not factored that calling cards do not factor in that you either need a landline, VOIP landline (which requires an internet connection) or VOIP to POTS service, both of the last items require an internet connection (typically ADSL in New Zealand) and have additional required costs.

Anyway, lets get back on topic, which was Ihugs/Vodafones preference for discounted PSTN to mobile calls.