The IP phones for the sales department reside in Subnet A, while the IP PBX resides in Subnet B of the same network. All network switches and routers are properly configured for each network. A…

The IP phones for the sales department reside in Subnet A, while the IP PBX resides in Subnet B of the same network. All network switches and routers are properly configured for each network. A technician visiting the sales department is having problems registering the sales department phones with the IP PBX. The technician can ping each phone from within Subnet A, but not from outside the subnet. Which of the following is most likely the cause of this problem?