[SR-Users] rport parameter in VIA header in uac remote registration

Hi, I'm currently trying to connect Kamailio, running in docker, to my FritzBox. The FritzBox is the modem provided by my ISP, with a builtin voip server which I can use to dial out (basically a PSTN gateway). In my setup I have a custom router/firewall/NAT between the FritzBox and the server. When I connect LinPhone to the FritzBox, from behind the NAT, it works fine. But when I use the Kamailio uac remote registration, I don't get a response. After some investigation with wireshark, I found that the the SIP response from the FritzBox comes on the wrong port, and gets rejected by the NAT. The request comes from port 1025, but the response is sent to port 5060. I compared the requests from LinPhone and Kamailio, and combined with some research, found it probably was the rport option in the VIA header. reply to, instead of the port indicated in the request, which would be necessary to effectively go through a NAT. After reading a lot of documentation (and even parts of the source), I wasn't able to find an option to enable this option. Is there some option I completely missed, or is it not possible? Or am I completely wrong in my "diagnosis"? Thanks in advance, Jetse