I struggled with this setup for a while using rtpproxy, i eventually went with rtpengine because it seems to have more features and is still maintained. 
I wrote a quick how-to (mostly for myself to remember)
https://blog.voipxswitch.com/2015/08/11/rtpengine-with-kamailio-as-load-balancer-and-ip-gateway/

Here is the kamailio.cfg 
http://blog.voipxswitch.com/wp-content/uploads/2015/08/kamailio.cfg-rtpengine.txt

On Fri, Oct 9, 2015 at 8:50 AM, Fred Posner <fred@palner.com> wrote:
On 10/08/2015 09:09 AM, Ovidiu Sas wrote:
> If the SDP is correct, then you might have specific issues related to
> your specific deployment case. Snippets from others config files won't
> help. You really need to investigate and understand your particular
> issue that you are facing and fix it accordingly.
>
> Regards,
> Ovidiu Sas
>
> On Oct 8, 2015 09:04, "Dirk Teurlings - SIGNET B.V."
> <dteurlings@signet.nl <mailto:dteurlings@signet.nl>> wrote:
>
>     On 30-09-15 13:29, Fred Posner wrote:
>
>
>         Without a version of rtpproxy using the -A flag, you'll need to
>         either
>         (1) update to a different version of rtpproxy or (2) skip
>         rtpproxy and
>         have your asterisk handle all the rtp.
>
>
>     I tried rtpproxy v2, with the -A flag in bridge mode ( -A
>     privateip/publicip ). This doesn't reflect anything in the SIP headers.
>

A is advertise which should be different from bridging, and not certain
why you'd be in bridged mode. Are you using more than one nic?

Normally, for -A use something like:

rtpproxy -A PUBLICIP -F -l PRIVATEIP -m 10000 -M 55000 -s udp:*:7722 -d INFO

(the -d INFO will increase your logging)

If you continue to have trouble, I'd recommend you post an ngrep of the
failure.

Fred Posner
http://www.palner.com (web)
+1-224-334-3733 (direct)

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