...when it's actually a circuit-switched call, that just uses an underlying packet bearer connection. So it's an IP "pipe", but the actual voice application is good old-fashioned telephony, without all the usual extra VoIP toys like presence.
I've talked before about CS voice over HSPA data bearers. I've also thrown cold water on the notion of using IMS Multimedia Telephony (MMTel) as the main long-term VoIPo3G / VoIPo4G service.
So there's an important question outstanding... if the long term trend is to move mobile towards LTE (and maybe mobile WiMAX).... how do we deal with voice? Will all phones and networks need to support GSM/UMTS in parallel, in perpetuity? It now seems clear that not all LTE networks will have an IMS core. Some will, but many will not.
My view is that there are three main options in the long term:
1) Some good (and ideally standardised) ways to implement non-IMS NGN VoIP applications over LTE. This doesn't need to be a dumb pipe approach, but could use assorted hooks for QoS, security and so on, if well-engineered.
2) Use LTE as a pipe, and put Skype's / Truphone's / Cisco's / BroadSoft's / whoever's VoIP over the top of it. Deal with issues like handover to other technologies in software in the servers and the handsets - perhaps under control of the operators, but perhaps not.
3) Develop some way to "tunnel" traditional circuit voice over LTE bearers, as we're already seeing with CS over HS.
Martin Sauter over at WirelessMoves has done some sterling work looking at this problem (he calls it the Voice Gap) recently. He's just posted on a proposal for Option 3, as well a related proposal for what's being called IMS Centralised Services.
The idea of re-using CS voice instead of a shiny new VoIP application is not new. Many fixed-NGN operators' VoIP services "look" like circuit-switched telephony to the end user. Many ordinary PSTN services have long transited "virtual circuits" in an IP-based transport network. UMA-based dual-mode VoWLAN does the same - the handset's telephony application remains unaware that it's being transported over WiFi.
Obviously, this is bad news for suppliers of VoIP application servers, whether IMS-based or not. They need to push hard to demonstrate the added benefits of "native" VoIP as an application, as otherwise they risk missing a big opportunity around LTE.