[Serusers] SER server: remote vs local

Hello, I have two SERs: one on local subnet (192.168.1.x), the other remote. I also have two exact phones: one with local SER, the other is on the same subnet with remote SER. Both of them can register correctly to the SER on the same subnet. However, my local IPphone fails to register to the remote SER. I am pretty sure the username and password are valid. To verify the username setup, I have xlite installed on the local network 192.168.1, it can register to both local and remote server without any problem by the same account. I notice that the successful registration always has port information in the header: Via: SIP/2.0/UDP 192.168.1.103:5060;rport=5060;branch=z9hG4bK8F05E1B1000941AC83B8205023AD0B8F The failed registration doesn't: Via: SIP/2.0/UDP 192.168.1.102;branch=z9hG4bKc0f9cc1144b12ca2;received=24.22.117.57 Can someone shed some light on it? thanks! steven