This application implements a ringtone service using early media. When the INVITE arrives to the application server a call leg against a media server is stablished. The media server plays a ringtone based on the caller identity. The sdp of the media server session is used to generate a 183 response (early media) before proxying the original request downstream.
When a final response arrives from the downstream the media server dialog is finished and the call stablished as usual.