May I ask what exactly is the problem..
Here is a sample policy from RFC 3960 (this doesn't exactly talk about 180
w/ SDP but suggests what a UAC can do when it receives early-media packets
in conjunction w/ a 180 response):
With this in mind, a UAC should develop its local policy regarding
local ringing generation. For example, a POTS ("Plain Old Telephone
Service")-like SIP User Agent (UA) could implement the following
local policy:
1. Unless a 180 (Ringing) response is received, never generate
local ringing.
2. If a 180 (Ringing) has been received but there are no incoming
media packets, generate local ringing.
3. If a 180 (Ringing) has been received and there are incoming
media packets, play them and do not generate local ringing.
Note that a 180 (Ringing) response means that the callee is
being alerted, and a UAS should send such a response if the
callee is being alerted, regardless of the status of the early
media session.
As far as I know, SDP and 18x handling is implementation dependant, though a
common use - that seems widely accepted - would be to have the SDP in 183
for ring back tone, when PSTN gateway is involved.
So if ur swicth provides 180 with SDP it should provide ringback tone or 183 with SDP.
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