It's unclear if VZW will allow direct SIP access. I doubt very much that you will ever be able to call a VZW customer by way of a SIP URI from any arbitrary ITSP.
They are moving to IP-based peering with other carriers. Once IP-based peering is more dominant then there's some likelihood that your call might find IP transit end-to-end and thus support HDVoice.
There are many different issue involved; call transport, namespace, signaling protocol, codecs, business models, compensation schemes, regulatory reality, political actions, Some of these situations are more likely to evolve, or evolve more quickly than others.--