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Collaboration, Voice & AV · Advanced

Early Media

Media exchanged before a call is answered, such as ringback tones or announcements, typically signaled by 183 Session Progress with SDP.

Definition

Early media allows the called network to play carrier ringback, busy announcements or IVR prompts before the 200 OK. In SIP it is usually triggered by a 183 Session Progress (or 180 Ringing) carrying SDP, often with PRACK for reliability. If early media is mishandled, callers may hear silence instead of ringback when calling out through the PSTN.

Example

A caller hears the carrier's number-not-in-service announcement through early media in a 183 response.

Exam tip

No ringback on outbound PSTN calls is often an early media or PRACK interworking issue on the SIP trunk.

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