What Optus is offering is a hosted IP telephony service (aka IP centrex): ie PABX type functionality is provided by equipment within the Optus network, rather than by equipment on the customers' premises and this functionality is accessed by signalling carried as IP packets from phones which send and receive voice as IP packets.
The ipPhone Premier service is provided over Optus' own DSLAMs, so Optus is able to manage QoS between the user's phone and the interconnection to the PSTN. The ipPhone Express service is provided over Optus ADSL services delivered by reselling Telstra ADSL services, so there is very little scope for Optus to manage QoS.
Optus described the service as "Groundbreaking". Hardly. Optus has been running an IP centrex services based on a Nortel platform but aimed at larger organisations for the past two years. Optus also said it was "the first large scale rollout of this product in Australia." Of this specific product perhaps, but for this type of service. certainly not.
And why did Optus simply call it VoIP, a term generally used for consumer grade services that are simply low cost replacements for a standard telephone line? More accurately the service is an IP telephony service.
One thing at least Optus got right: "While VoIP is an area of much market hype, steady growth is occurring among businesses with existing IP data networks." But with 14 occurrences of 'VoIP' in a 450 word press release and little hard information, Optus seemed to be doing its best to add to this market hype.