A session border manager (SBM) may receive a session initiation protocol (SIP) request for a session or a service involving a first user equipment (UE) that is subscribed to a first network and a second UE, that is subscribed to a second network, wherein the SIP request indicates a service type of the session or the service. The SBM may determine, based on the service type, a subnetwork of the second network that is to facilitate the session or the service, wherein the subnetwork is one subnetwork, of a plurality of subnetworks, of the second network that is designated to manage communications that are associated with the service type. The SBM may transmit the SIP request to the subnetwork to cause the subnetwork to initiate the session or the service with the second UE. The SBM may receive, from the subnetwork, a response to the SIP request.