Asterisk-Lync RTP Unknown version 0

Hello everybody,

I’m sending you this email because I have a little problem with my Asterisk-Lync configuration.
I’m trying to get Asterisk work with Microsoft Lync (OCS), but I only have RTP stream in one side.
I installed wireshark in my mediation server (Microsoft Lync) and I can see that I get the error : RTP Unknown version 0 (Version old VAT Version).
I have RTP only from Asterisk side (Asterisk -> Lync), and not from the Lync (Lync->Asterisk) side because he didn’t understand the RTP packet and send me a RTP Unknown Version 0.
Have you any Ideas to resolve this problem?
Some people told me that can be a STUN problem, but I have no Idea about how to resolve the problem.

Thank you very much!