[Serusers] SER and RTPproxy

I Think I have thing configured correctly and yet I can not make things work correctly. This is what I am do I have SER on an internal (10.1.1.x) IP address I have RTPproxy on a different machine with a public IP address 206.75.15.x I am routing the call out over our firewall and things 'almost' work. SER sets up the call using the RTPproxy to set up the ports for the RTP stream, and the RTP stream is returned from the far end to the public IP address, however non of the RTP traffic is sent to the internal address, I am guessing it is waiting for an rtp stream from the internal address but neither SER nor the ip-phone send anything or even seem to know about the RTPproxy. Can anyone offer ANY suggestions as to how to direct the internal RTP stream to the RTPproxy address? Many thanks, Jeffr