Sean,
I am not sure about the doc. But you can use the following way:
Scenario:
A->PA->PB->B
On receiving the INVITE PB should send out one INVITE with no SDP to
MMS. MMS will return the SDP(MMS) in 200 OK. Use this SDP to send a
INVITE to B. B will send its own SDP(B) in 180 ringing.(I think your
phone should be capable of receiving the media even the user is
hang-up). Send an Update to MMS containing the SDP(B). Then start
playing the sound on B through MMS. Meanwhile send 180 ringing to PA
with B's SDP.
When B reports 200 OK( i.e user has answered) generate an UPDATE in PB
and send it to B with A's SDP. The A will send a 200 OK for it. Also
forward 200 OK for it. See below
A PA PB B MMS
---Invite(A)----------------|------------Invite(No SDP)------>
<-------100 trying----------|<------------200 OK(MMS SDP)-----
|--Invite(MMSSDP)->|
|<--180 R(B's SDP)-|
<-------180 Ringing---------|---------->Update(B's SDP)---------->
................Listening though MMS............................
|<--200 OK-------|(Replied)
---------200 OK(A's SDP)----|---Update(A's SDP)-->|
|----------Update(SDP = 0)-------->
Regards,
Rahul
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