VOLTE: Location information in S8HR

Question

in case of VOLTE roaming integrated in S8 Home-Routing, does the subscriber location information contained in the SIP message generated from the UE?
If not, how the S-CSCF obtain this information?
Is it by querying the HSS (Diameter Cx interface)? if yes when? on SIP registration or per each call attempt?
Is there an option that the S-CSCF will not be aware on the subscriber location and the TAS shall obtain it by querying the HSS (Diameter Sh interface)

Answer 1

I would like to share my understanding as well as I have few queries.

UE initiates IMS call when attach has already done. As part of attach procedure, UE location is already update with HSS. So Home HSS already knows which RAT and PLMN is serving the UE.
Now I have question why S-CSCF would be interested to get UE location details ? As per my knowledge, S-CSCF exchanges "Server-Assignment-Request and Server-Assignment-Answer messages" with HSS to store the information in HSS which server is serving the user for IMS. Later on when I-CSCF needs to fetch serving S-CSCF for an UE, it retrieves the serving S-CSCF information from HSS and proceed with further signalling.
Application Server (AS) can fetch the details of UE from HSS using Sh interface. In User-Data-Request message, AS can ask for location of UE. If HSS supports feature to fetch the active location from MME, then it will fetch and provide the current location of UE to AS in User-Data-Answer.
However, I am still searching the AVP in the specification by which MME updates HSS for UE radio locations (in terms of Cell-Global Identifier and Tracking area code).