Confirming that voice continues to be one of the hottest topics in the LTE space, the venerable Total Telecoms has published a feature on the topic. Skip ahead to pages 14/15...
My favorite quote is from Nokia/Siemens:
"VoLGA is a proprietary solution. It needs a specific box to be added to the network; you can't do rich voice services, combining voice and multimedia."
I think "proprietary" is a bit much. The VoLGA specs are available for anyone to download from VoLGA Forum website, so they aren't particularly secret. Plus the specs are completely based on the existing 3GPP GAN spec.
The specifications were actually developed by a group of companies -- open and available to anyone interested in participating. I believe NSN was invited to contribute as a VoLGA Forum member and chose not too.
As for a 'specific box'… all solutions are going to require new 'boxes' (plural). The fact that VoLGA is just a single box is actually an advantage.
Finally, NSN suggests that VoLGA 'can't do rich voice…', perhaps they didn't read the RCS 1.0 specification, which specifically called out the support of circuit voice + IMS/RCS data. Even the IMS specifications don't require SIP/VoIP, it was designed from the beginning to support CS, and therefore supports VoLGA.
Of course the alternative, CS Fallback, has no hope of ever supporting combinational services…