Early media is simply media that is sent before a call is answered. It’s not the voice of the person you called, but rather system tones, announcements, or any other sound that the phone company wants to send your way. It’s the distinctive ringing you hear when you call a telephone in England. It’s an “all circuits are busy” message. It’s anything you might hear until you hear the called party’s voice.
Early media is typically supported by the use of the 183 Session In Progress response. Unlike a 180 Ringing response, 183 will contain SDP. This SDP is used to establish a media connection that carries those network tones and messages. It will eventually be torn down when the call is answered, but until then, it’s a way for the caller to audibly hear call progress.
I have been talking about early media in terms of the public switched telephone network (PSTN), but early media is also used by some IP PBXs. Why? Because they want to play the same kinds of sounds that the PSTN uses. They want to play announcements and country specific ring-back. Remember, PBXs are rooted in the same TDM world as the PSTN and have adopted much of its behavior.
https://andrewjprokop.wordpress.com/2014/04/18/sip-media-management-early-media-vs-late-media/
https://tools.ietf.org/html/rfc3960
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