I set up openser. It works perfectly if I send the traffic to a PSTN gateway, I have sound both ways, no problem. If I do a sip call to an other sip phone, I have no sound, both ways. It only works if both sip phones are in the same network (even if the SER server is in the web). I think the connection for the voice goes directly from sipphone to sipphone. Can it work without a mediaproxy?