An architecture and procedures by which an NGN can provide the following capabilities to an NGCN user: basic terminating service in the peering model; enhanced terminating services in the peering model; enhanced originating services in the peering model; and roaming service in the subscription and peering models. All capabilities are provided in an easily scalable manner via provisioned subscriber data and small modifications to existing IMS procedures. In the first case, an NGN is configured among the external networks to receive all SIP requests intended for the NGCN users of an NGCN interconnected to the NGN via the peering model (this NGN is the trunking home NGN), and the NGN forwards these requests to the NGCN for handling using request retargeting. In the second case, an NGN is configured among the external networks to receive all SIP requests intended for the NGCN users of an NGCN interconnected to the NGN via the peering model, and the NGN applies a set of agreed services using NGN ASs (application servers) to each request before forwarding to the NGCN for handling. In the third case, the NGCN is configured to forward all originating requests to an NGN (the trunking home NGN) via the peering model, and the NGN applies a set of agreed services using NGN ASs to each request before forwarding to the intended recipient. In the fourth case, an NGN is configured among the external networks as the roaming home NGN to receive a SIP REGISTER request from an NGCN user that has roaming capability, and the NGN forwards that request to the NGCN for handling.