[Serusers] OptiPoint420 behind a NAT device

Hi all, i have a ser setup (CentOS 4.4/ser 0.9.6) with several ip phones (SNOM320 and Xlite30) and every thing seems to work fine. The ser server has a public ip address 89.xxx.xxx.xxx and the ua are behind a NAT device (corporate FW). I want now to add some OptiPoint420 SIP but was unable to get them working. That means that the registration is ok and calls to the Optipoints from other ua (xlite or Snom) work but i was unable to place a call from Optipoint to other ua. In the trace you can notice that the INVITE is sent to ser with Src Port 38625 and Dst Port 5060. But the response from ser is sent to Src Port: 5060 (5060), Dst Port: 5060 (5060). The result is of couse ICMP Destination unreachable (Port unreachable). I have rtp proxy and ser running on the same server and my ser.cfg looks like the one from the SER GettingStarted http://siprouter.onsip.org/doc/gettingstarted/ch08s02.html). I read the doc about "handling of NAT using RTP Proxy" but was unable to change the config to get this scenario with OptiPoint working. Has anyone managed to get OptiPoint to work with ser? thx in advance? No. Time Source Destination Protocol Info 14 8.907171 89.xxx.xxx.xxx 89.xxx.xxx.xxx SIP/SDP Request: INVITE sip:8001@registrar.mydomaine.com;transport=udp, with session description Frame 14 (972 bytes on wire, 972 bytes captured) Ethernet II, Src: AminoCom_02:02:02 (00:02:02:02:02:02), Dst: AcerTech_9c:00:8d (00:00:e2:9c:00:8d) Internet Protocol, Src: 89.xxx.xxx.xxx (89.xxx.xxx.xxx), Dst: 89.xxx.xxx.xxx (89.xxx.xxx.xxx) User Datagram Protocol, Src Port: 38625 (38625), Dst Port: 5060 (5060) Session Initiation Protocol No. Time Source Destination Protocol Info 15 8.907394 89.xxx.xxx.xxx 89.xxx.xxx.xxx SIP Status: 407 Proxy Authentication Required Frame 15 (772 bytes on wire, 772 bytes captured) Ethernet II, Src: AcerTech_9c:00:8d (00:00:e2:9c:00:8d), Dst: AminoCom_02:02:02 (00:02:02:02:02:02) Internet Protocol, Src: 89.xxx.xxx.xxx (89.xxx.xxx.xxx), Dst: 89.xxx.xxx.xxx (89.xxx.xxx.xxx) User Datagram Protocol, Src Port: 5060 (5060), Dst Port: 5060 (5060) Session Initiation Protocol