as the news link stated, NOW is strictly Digi to Digi where both end are iPhone VoLTE capable phones. then and only then the VoLTE will go through.

the VoLTE features has to be supported at the LTE's EPC, the equipment is called IMS, within digi own LTE network, which is ZTE, i believe the IMS is also belong to ZTE, i m not sure on this. if it is ZTE, ZTE only need to maintain their own IMS and solve the VoLTE problem if any issue arises. but when you are talking about Inter-Telco VoLTE call, the IMS of telco A will have to route to IMS of telco B to make VoLTE possible, I am not if there has been live or not, I didnt follow this news closely though.

How can a phone will know who he dial is iphone with volte capability or no before reaching them?
--------a phone with VoLTE capablity will have a tag of this features when making the call setup, when the EPC receive this VoLTE tag, it will check if the destination caller is also on the LTE network and support VoLTE, if it has, then the call will patch through on 4G network, else it would be fallback to 3G or 2G depending on the signal conditions.

volte quality to me is VoLTE features, but the news link claimed it is HD-Voice, which 3G already can support HD voice when circuit switch can handled AMR-WB codec of 12.65Kbps.

as for VoLTE is double the bitrate, or 23.85kbps to be more precise. Calls will sound even clearer, and that is the highest possible bitrate defined in 3GPP standard for VoLTE. which i believe is also the same bitrate for VoWIFI