I have tested cisco ATA186 (private IP) with SER under different circumstances. With Linksys and D-link router, there were no problem. But with Lioncom ADSL router, a strange thing happened. In this case, the ATA is having a private IP 192.168.5.2 and the SIP port number is the default 5060. The public IP of the ADSL connection was 195.226.252.124. The following is the traces from ngrep. As you can see, the Lioncom router forwarded (NAT) the SIP REGISTER with port 30150 while SER sent the response back to port 5060. So the ATA couldn't receive the response at all. What maybe the problem?

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U 195.226.252.124:30150 -> 192.168.3.29:5060
REGISTER sip:sip.abc.com SIP/2.0..Via: SIP/2.0/UDP 192.168.5.2:5060..From: <sip:6701892@si
p.abc.com;user=phone>..To: <sip:6701892@sip.abc.com;user=phone>..Call-ID: 1781909739@1
92.168.5.2..CSeq: 1 REGISTER..Contact: *..Expires: 0..User-Agent: Cisco ATA v2.15 ata18x (0209
27a)..Content-Length: 0....
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U 192.168.3.29:5060 -> 195.226.252.124:5060
SIP/2.0 401 Unauthorized..Via: SIP/2.0/UDP 192.168.5.2:5060;received=195.226.252.124..From: <s
ip:6701892@sip.abc.com;user=phone>..To: <sip:6701892@sip.abc.com;user=phone>;tag=d907c
037823644515dfe0ede38ca9976.9b53..Call-ID: 1781909739@192.168.5.2..CSeq: 1 REGISTER..WWW-Authen
ticate: Digest realm="sip.abc.com", nonce="3e89f40b00000000e3fd8eedb58e5662ca62981749bb0006
", algorithm=MD5..Server: Sip EXpress router (0.8.10 (i386/linux))..Content-Length: 0..Warning:
392 192.168.3.29:5060 "Noisy feedback tells: pid=10613 req_src_ip=195.226.252.124 in_uri=sip:s
ip.bmx-inc.com out_uri=sip:sip.abc.com via_cnt==1"....