The Cisco 1751 router can support three 4-port Foreign Exchange Station (FXS)/Direct Inward Dialing (DID) voice interface cards (VICs) and up to a maximum of four DID ports.
The Cisco 1760 router can support four 4-port FXS/DID VICs and up to a maximum of eight DID ports.
The 4-port FXS/DID VIC provides both FXS and DID functionality on a single VIC. You can select either FXS or DID mode on a port-by-port basis. The default mode is FXS.
Refer to Configuring VIC-4FXS/DID and Understanding Foreign Exchange Station (FXS) Voice Interface Cards for more information.