[Serusers] NAT with SER and ASTERISK, strange behaviour

Hi folks, I just spent the evening with trying to find a logical solution to a NAT problem but I had no success. I decided therefore to go to bed and let the more intelligent guys have a guess ;): I do have the following setup: Cisco 7960G with public IP Xlite behind router with NAT Ser server with a public ip Asterisk with a public ip Xlite finds out that it is behind a symetric firewall while starting and sends the external ip of the nat device in its sip messages. I want to call the xlite client behind NAT from the Cisco phone. It works when the cisco phone and the xlite client are registered with the asterisk server. I have set nat=yes and canreinvite=no in the account settings for the xlite client in the sip.conf of the asterisk server. The Cisco phone has a normal account. I had a look at the sip messages and as configured, the Asterisk server works as a rtp proxy for the media stream. Audio works in both ways. When I use ser and register both clients with ser and start a call, audio works only outbound from xlite to the cisco phone but not inbound from the cisco phone to xlite through the nat device. I digged the sip messages several times and did not see any big difference between the two sip conversations, besides having the asterisk as an rtp proxy in the middle in the first attempt. Neither did I see an approach of asterisk to make xlite pinhole the nat device as describe for example in the sip cookbook. Unfortunately I am stuck at this stage and cannot find a logical explanation for the described behaviour. If anyone can give me a hint or needs further information to assist, please let me know! Cheers Morten