This option is applicable to SNPNs not providing own IMS services. It enables the SNPN to use IMS services from an independent IMS provider, e.g. PLMN or another entity, which is the ISH, providing IMS services for SNPN subscribers. It is assumed that the SNPN has an agreement with the ISH. This agreement describes e.g. P-CSCF address provisioning and security aspects for all standardized reference points that are implemented between both administrative domains. In this deployment option either IMC or USIM/ISIM credentials are used to authenticate and authorize a UE for IMS services. The UE holds an association between an SNPN subscription and the corresponding IMS subscription to ensure proper credentials are used to access SNPN and IMS services in this deployment option. The mechanism how this association is stored in the UE is out of scope of 3GPP. A potential architecture for this deployment option is depicted in Figure AB.1.2.1-1.

Support for Emergency Services requires that the SNPN, based on an agreement with the ISH, broadcasts IMS Emergency Services support as described in TS 23.501.

In order for an SNPN to enable IMS services to be provided via multiple independent ISH, it is required that for every SNPN subscriber, the IMS DNN stored in UDM of the SNPN contains the information, which is used to identify the corresponding ISH, e.g. ISH domain name is encoded as part of IMS DNN. At PDU session setup, the SMF selects the P-CSCF of the ISH based on clause 5.16.3.11 and clause 5.16.3.4 of TS 23.501. The CH and SNPN need to agree that the AMF in SNPN that receives the subscription information that includes the IMS DNN and contains information to identify the corresponding ISH, is able to determine the ISH and therefore derive correctly the IMS voice over PS Session Supported Indication as defined in clause 5.16.3.2 of TS 23.501.