This is probably because you are sending and receiving calls within the same card. When only one card is used, the system is aware of that both the originating and terminating end points are sitting in the same unit. With that, the packets will be routed internally between the different applications, instead of routing them outside. This is normal. If users would like to see the packets being sent out of the network interface and routing back, then they must have a software option called "2-port". With this option, originating end points configured in port 0 of the card will be able to send packets from the network interface port 0, and routed back (via an external router / switch) to the terminating end point in port 1.