The 4-port high-density FXS/DID VIC can support up to 4 foreign-exchange-station (FXS) ports for directly connecting phones or fax machines, or it can be used to connect up to four direct-inward-dial (DID) analog trunks, providing customers the flexibility they need for their unique business environment. Each port on the 4-port FXS/DID VIC is selectable for use in either FXS or DID mode. The 4-port FXS/DID VIC is the same size as the existing two-port voice interface cards (VICs), allowing it to slide into either WIC/VIC slot or VIC slot in Cisco 1751 and 1760. The Cisco 1751 can support three VIC-4FXS/DID cards with a maximum of four ports in DID mode. The Cisco 1760 can support four VIC-4FXS/DID cards with a maximum of eight ports in DID mode. The VIC-4FXS/DID in the Cisco 1700 is used to connect analog phones or fax machines. The long-distance calls between small enterprise branch offices and headquarters are carried by IP network, while the local calls are carried by PSTN, which also provide backup to the IP network.