[Serusers] with private IP , I cann't hear the one with public IP

hi everyone! I have installed ser-0.8.10 and ser-mysql-0.8.10 on Red Hat Linux 7.2. After I made a little configuration on the script (even did not add anything). The SIP server can work well in LAN .There are Two telephones both in the LAN can get though. When I put ser on Internet, and put the two telephones on Internet with their own public IP addresses, they can work well too. Then I put one in a intranet with its private IP address, and the other on Internet with its public IP address, they can get through too. with the telephone with public IP ddress I can hear the other, But the other cann't. What's wrong? I have no idea what I can do . Anyone help me please ! Thank you! 　　　　　　　　gg 　　　　　　　　gg 　　　　　　　　gdut200104024532(a)163.com 　　　　　　　　　　2004-08-20