A telecommunications service provider provides a telecommunications infrastructure that is based in part on an IP Multimedia Subsystem (IMS). The provider may have various IMS application servers to support different services for subscribers of the provider, such as messaging, voice communications, presence, etc. In addition, third parties may provide their own IMS application servers for the same services or for different services. Subscribers of the provider are routed to the provider-supported application servers, while subscribers of the third parties are routed to application servers of the third parties. This allows third parties to implement and control their own services, while also leveraging the existing infrastructure of the provider.