When TAS gateway tries to register some extensions of Cisco PBX, only the port which registers extension on priority is able to register it successfully, other ports will not be able to get registered.
PCAP file captured from gateway, it could see when the other ports was trying to register extension, gateway sent REGISTER sip packet accordingly to PBX while Cisco PBX replied it with 404 Not Found instead of expected 401 authentication.
Actually it is the policy that Cisco PBX considers it as same extension if REGISTER packet is from same IP and same port. In this case, all REGISTER packets are from same ip 10.1.3.20 and same sip port 5060 to PBX, so we might make the packets be from different sip ports.
Enable Random Port on FXS gateway.
On TA100&TA200, the option is under FXS port -> Advanced tab -> Enable Random Port.
On TA400 or above, the option is under VoIP Server Settings -> Advanced tab -> Enable Random Port.