Да, все правильно. Только смотреть лучше в TS 24.229 п.5.1.2A, т.к. заголовок Service-Route -это расширение введенное для нужд 3GPP сетей.
IMS возвращает в Service-Route адрес(a) S-CSCF, который должен быть вторым в списке Route (может быть несколько URI в Service-Route, тогда все по порядку, т.е второй и дальше). UE при формировании запроса non-REGISGTER заполняет Route всегда так: первый URI в Route -это P-CSCF (Outbound Proxy), дальше URI S-CSCF, сохраненный из заголовка Service-Route в SIP 200 OK на REGISTER.
P-CSCF (Outbound Proxy) в случае не-мобильного клиента либо настраивается в конфиге, либо выдается через DHCP.
>>
The UE shall build a proper preloaded Route header field value for all new dialogs and standalone transactions. The UE shall build a list of Route header field values made out of the following, in this order:
a) the P-CSCF URI containing the IP address acquired at the time of the P-CSCF discovery procedures which was used in registration of the contact address (or registration flow); and
NOTE 15:If the UE is provisioned with or receives a FQDN at the time of the P-CSCF discovery procedures, the FQDN is resolved to an IP address at the time of the P-CSCF discovery procedures.
b) the P-CSCF port based on the security mechanism in use:
- if IMS AKA or SIP digest with TLS is in use as a security mechanism, the protected server port learnt during the registration procedure;
- if SIP digest without TLS, NASS-IMS bundled authentication or GPRS-IMS-Bundled authentication is in use as a security mechanism, the unprotected server port used during the registration procedure;
c) and the values received in the Service-Route header field saved from the 200 (OK) response to the last registration or re-registration of the public user identity with associated contact address.