But currently the call will be like this.
For example DiGi user with volte call to YES 4G VoLTE
DiGi user --> DiGi IMS -- > DiGi 2G/3G network ---> YES 4G Gateway -- > YES 4G IMS --- YES 4G user
and both side will using VoLTE 100000000% for sure.
DiGi IMS network connects to other IMS network will be real after this, for better call setup advantages and saving TDM costs.
FYI, government previously still forcing telcos to use TDM for interconnection between telcos.
FYI, ZTE is ZTE. Many issues comes due to legacy 2G/3G/CSFB network. I cannot tell more at here

A big wrong.
EPC/PGW detect this when user register VoLTE via different APN (which is "ims" for U Mobile, DiGi and Yes 4G, im not sure whether this is standard APN for all VoLTE in the world or no). Then phone will perform SIP registration to IMS network.
Calling will cause phone trigger SIP calling to ANY numbers, as long as the number exist (999 and 112 are still pending MCMC standard).
VoLTE and Wifi calling all using SIP call. A SIP call wont check where is the destination or even the destination is fix line, or mobile or VoLTE or no. The call just go through.
If i am U Mobile and U are digi, and u can know my subscriber using VoLTE or no then GG ad lo me
but end to end HD voice capability can work if both parties agree on this
For example, DiGi VoLTE users ---> DiGi IMS ---> DiGi 23G ---> UM 3G ---> UM users (who phone support HD voice)
But ... slow slow wait.... government very slow at this.
talking about SIP call,
Did u see any skype call cant reach fix line/mobile phones?
Did u see any voip calls cant reach fix line/mobile phones due to they are SIP calling?
All legacy networks are accepting SIP INVITE and processing accordingly.
Those calls between MSCs and between OLOs are now using SIP protocol too(this one seldom, will implement soon).
And im holding iphone 6. I can call anyone using VoLTE (my signal wont drop to 3G).
And also Im part of the VoLTE testing. Im sure how DiGi core network/IMS network works.
Im not sure bout this... but normal call is 64kbps dedicated bandwidth... i think HD voice is more to the kHz the audio is captured? even though 3G HD voice also got 6.xx kbps
Currently digi maxis umobile celcom using 12.65kbps for HD voice codec bit.... and UM normal call using 12.2kbps... tat is y UM calls will be only slightly difference between HD call and normal call in its 3G network
https://patentimages.storage.googleapis.com...000025_0001.pngthanks for your explanation on the IMS part, I wasn't really look into the SIP part and the interconnect between telcos in term of VoLTE, i need to find out more on this topic then reply.