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FXS Foreign eXchange Subscriber interface (the plug on the wall) delivers Plain Old Telephone Service (POTS) from the Central Office (CO) of the local phone company and must be connected to subscriber equipment, such as telephones, modems, and fax machines. In other words an FXS interface points to the subscriber. An FXS interface provides these primary services to a subscriber device:

  • Dial Tone

  • Battery Current

  • Ring Voltage

The FXS acronym can also be rendered as Foreign eXchange System.

FXO Foreign eXchange Office interface (the plug on the phone) receives the POTS, typically from a CO of the Public Switched Telephone Network (PSTN). In other words, an FXO interface points to the Telco office. An FXO interface provides this primary service to the Telco network device:

  • on-hook/off-hook indication (loop closure)

A registered jack (RJ)-11 connection is used in order to connect to an FXS interface, such as the PSTN or a Private Branch eXchange (PBX). This is the interface a standard telephone set provides. It uses only two wires (tip and ring) for signaling and audio. It employs a relay that is closed for off-hook and open for on-hook. FXO ports on Cisco IOS  Software platforms can support both loop-start and ground-start signaling. Ground-start signaling is used primarily on trunk lines or tie lines between PBXs.

An RJ-11 connection is used in order to connect common line side equipment such as a phone. It uses only two wires (tip and ring) for both signaling and the audio path. It supplies direct current, loop current, dial-tone, and alternating current ring voltage. It must connect to an FXO interface with loop-start or ground-start signaling. Standard residential phone lines are configured as FXS loop-start.

Loop-start is the default signaling on Cisco IOS FXS and FXO voice ports. In order to change it, issue the signal ground-start voice-port command. Reset the voice-port after any changes with the shutdown/no shutdown command sequence.

Refer to these documents for more information:

Common issues with the analog FXO and FXS circuits


Is there a command in which you can shut and no shut the FX0 and FXS modules (reset the cards) without taking down the router (c2911)? Rigth now we use a VG224 with a C2911. If anything goes wrong with the analog lines we can reboot the VG224 without effecting the C2911. That way the office stays up and ruuning. But we don't need all 24 ports from the VG. If we can reboot just the module in the C2911 we can remove the VG.


thanks, there are two columns dnormask and dnderived where the alternate number seems to be stored. In my CUCM, both contain the same value. Not sure if I have to update the new alternate number in one or both columns. But I can do trial and error to figure that. Many thanks for your help.

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