We have several VoIP devices (X-Lite, Polycom phones) with private IPs NATed behind a RB3011UiAS talking to two of our public IP VoIP servers. Server1 is a proxy for SIP traffic only and does not handle any RTP. SIP traffic is forwarded from Server1 on to Server2, and RTP is negotiated to pass directly between the client (Xlite) and Server2. In this case, the client and Server2 negotiate a set of UDP ports to use for RTP, but when Xlite starts sending RTP to Server2 the NAT uses (typically) port 1026 as the source port on the Mikrotik. This source port (and listening port) are not expected by Server2 so there is no audio in either direction. We have found that if we proxy RTP through Server1 instead of allowing it to route directly to Server2, the NAT in mikrotik honors the original port selection (not 1026, usually 50000+) and audio is heard in both directions. We do not prefer to have RTP proxied through Server1 unless we have no other choice. Does this scenario sound familiar to anyone?