[Sip] Sending an announcement!!

[Kolinowitz Bernd <bernd.kolinowitz@kapsch.net>]

Hi!

We want to implement a typical PSTN feature in our SIP proxy. If caller A sends an INVITE (including audio in the sdp) to the sip-proxy, the proxy should send an announcement (like a jingle) to caller A while it is trying to reach the callee.

We are now searching for a way to implement the feature concerning RFC 3264 (offer/answer model of SDP). First we thought to answer with early media, that means to send a 183 response including the SDP for the announcement and demanding a PRACK request.
Next we have thought to forward the 200 response from the callee to the caller. 
Now there is our problem: What should the caller do with the SDP?

The good version: He accept the SDP from the callee as an answer of his SDP offer in the first INVITE-request and sends an empty ACK.
The bad version: The SDP in the 200 response will be treated as a new offer and he sends an SDP answer in the ACK request.

How will the client react? Is there another way to realize this feature?

Thanks in advance
Bernd


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