Hi,
mhomed did the trick nicely for ser, but force_rtp_proxy is putting the wrong interface in the c= bit of the outgoing INVITE. c= is getting the IP address of the internal interface, not the external. There doesn't seem to be any way of working around this at the moment, or am I missing something?
Unless someone has something lying around to solve this I don't mind working on it myself, unless I am missing something and this is a particularly hard thing to sort out.
I suspect there are ways round this by masqerading on the ser box but I don't want to do that and would much rather not have to enable ip forwarding on the box at all.