This clause depicts the originating procedures for IP-PBXs using static mode business trunking as described in TS 24.525
when the IP-PBX is not provisioned as a subscriber in HSS and served via the Transit Function.
The following steps are performed:
An enterprise user within the IP-PBX tries to establish a call. The IP-PBX sends an INVITE towards IMS via the IBCF (contact point for the IP-PBX). If no security association exists between the IBCF and IP-PBX, TLS will be initiated as a result of trying to send the INVITE. Once the TLS session is setup (using the certificates), the INVITE will be sent over the secure connection. The INVITE is assumed to include a calling party identity.
The IBCF may apply general screening rules to the request, and adds a P-Served-User-Identity to the INVITE with the identity of the IP-PBX (SIP URI identifying the domain of the IP-PBX retrieved from the certificate). Additionally, the IBCF adds the orig parameter to the INVITE to indicate that this is an origination request. The IBCF sends the INVITE to the I-CSCF).
The IBCF performs the same actions as in step 2a. but sends the INVITE directly to the Transit Function instead of the I-CSCF. (The next step is step 5).
The I-CSCF performs the normal (originating request) user location request towards HSS to find the served user, but as it is not provisioned in HSS, "user not found" is returned.
The I-CSCF sends the INVITE to the Transit Function.
The Transit Function is configured with a set of Transit invocation criteria that are triggered to find a correct AS to route to. As this is an origination case (as indicated by the orig parameter), the P-Served-User-Identity is used to identify the IP-PBX.
The Transit Function forwards the request to the AS hosting the IP-PBX identity assertion. Based on the P-Served-User-Identity, this AS identifies the IP-PBX, verifies that this IP-PBX is a valid user and inserts a P-Asserted-Identity identifying the enterprise user. Other ASs may be triggered based on iFC and may, based on the P-Asserted-Identity, apply any enterprise specific actions if required.
The INVITE is forwarded for further onward routing towards the remote side.
Transit Function performs onward routing towards the remote side.
The session setup is completed.