[Serusers] Does SER works on TWO network interfaces with public and private IP addresses?

Hi, ALL: I use ser + mediaproxy + PSTN support, and my ser with two interfaces. One is public IP address such as 211.21.xxx.xxx. Another one is private IP address such as 192.168.11.1. And I use XLite (192.168.11.2) register to SER's private interface(via HUB only). It can register sucessfully. But when I make a call to PSTN with this XLite, the callee rings and I answer it. I can not hear any sounds from each side. I try to register another XLite(192.168.11.3), and make a call to another private XLite(192.168.11.2). I can hear rings but it is still no any sounds from each side. Can anyone tell me what it happens? Best Regard Charles