Cliënt <MSlync> behind NAT can't hear announcement before answer!

Case:

Apathy <behind a NAT> is not receive the announcement before 200 OK <answer>, because own sbc is waiting that A Party
to start send rtp <source ip/port>. The config have symmetric latching enabled!

Apaty behind a NAT<====================>Bparty
|=============== |         |==========|         |===================|
| MSlync client          |====| SBC             |=== =|IVR Bnumber 0900XX    |
|-------------------------|        |-----------------|         |------------------------------|
INVITE+SDP->                       |    -> INVITE+SDP
                       <-Trying          |                         <-Trying
                        <-183+SDP    |                         <-183+SDP
PRACK->                               |    ->PRACK
                <-200OK<PRACK>|                          <-200OK<prack>
stunt message->                    |
stunt message->                    |
stunt message->                    |
stunt message->                    |
/////no RTP stream////              |<==============RTP stream flows <announcement are play by bsite>
                                               |
                                               |
                       <-200OK          |                           <- 200OK <answer>
                                               |
====>RTP start to flow           |===============>RTP start to flow