Sorry for the late answer.
Lot of points in this thread/post.
Most CSPs – Communication Service Providers – have deployed 4G / IMS for VoLTE and ViLTE Services.
In LTE architecture, core network includes Mobility Management Entity (MME), Serving Gateway (SGW), Packet Data Network Gateway (PGW) and Enhanced NodeB (eNodeB).
3GPP can be used to look at the signalling / protocols. In particular, you can find information using Google and www dot 3gpp dot org.
Key areas to look at: L1, L2, SCTP, IP, S1-AP, X2, etc..
At the time of writing 5G is now being introduced.
3GPP TS 29.281 – GTP
3GPP TS 36.410 – S1 Layer 1
VBR/ Wallis Dudhnath