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British FXO / FXS card supporting standard BT (British pin out)

Level 3
Level 3

Hi all,

On my wall outlet I have a socket which uses the outer 2 pins of the wire for the connection to the exchange (FXO). I have made a cable to route this to the inner 2 pins to get this to work. Is there a config option to allow me to do this internally within the FXO card?

Similarly, to get back to a standard BT store-purchased phone (wireless) I need to use another cable which converts the inner 2 pins to the outer 2 pins as it connects to the phone. Can my FXS card do this internally with a command?

I had this all working at my old home wit just cables... now I need to make two RJ11 cables up. All I can think of is either,

- the IOS I'm using is missing something

- I am missing something

- or the PSTN mutli-plug I was using did this conversion for me without my knowledge.

I thought, "CPTone GB" did this conversion, for some reason...

Any ideas?

4 Replies 4

paolo bevilacqua
Hall of Fame
Hall of Fame

Voice interfaces do not change pinout by configuration, You will need to have the correct cable.

Level 7
Level 7

Hi Tony

To confirm, the standard Cisco FXO/FXS cards work just fine with both BT lines and other devices. I use standard RJ-11 to Telco cables, and they work fine.

One thing - for your FXS card, you may need an adapter which provides "PBX master" function as per this:

HTH. Barry

Barry Hesk

Intrinsic Network Solutions

FXS Ring Failure in the United Kingdom

A telephone approved for the United Kingdom might fail to ring when connected to a Cisco FXS port. The failure results from a physical interoperability issue and is independent of Cisco hardware or software. British Telecom did not implement RJ-11 type connectors when it adopted plug-and-socket connection methodology. RJ-11 connectors allow parallel connectivity for the transmission path and the ringer circuit. They were not used because older telephones needed to have their ringer circuits connected in series due to a requirement for high current.

Outside the United Kingdom, ringer circuitry is self-contained in each phone. The U.K. implementation puts the capacitor, which provides the AC ring path, and the antitinkle feature (prevents the bell or ringer from sounding when pulse dialing is used) externally in the first socket, connected to the local loop.

In the United Kingdom, certain British Approval Board for Telecommunications (BABT) telephones fail to ring when they are connected to FXS ports on Cisco voice-enabled routers and switches. Outgoing calls can be made and voice communication in both directions can be established. However, incoming calls do not ring the telephone. These telephones functioned correctly before they were connected to the FXS ports.

Because a proprietary connection system is implemented, you must use an adapter to connect the telephone to an FXS port. The adapter must be a master that contains the capacitor, or the telephone fails to ring.

For a schematic and more information, refer to Understanding Why Telephones in the United Kingdom Connected to Cisco FXS Interfaces May Fail to Ring, document ID 25800.


This is a ppt slide I put together for FXS/FXO in the UK.

Hope it helps

Please rate useful posts.

Regards, Alex. Please rate useful posts.

Level 3
Level 3

Hi all thanks for the responses!

An update, after making up two RJ11 / BT cables for my FXO and FXS I found my cable which I used at my other home. I plugged it into the BT socket on the wall from the telco and plugged the RJ-11 side into my FXO port.. All works! I looked at the RJ11 side to compare the pin-outs and it looks as though the green and the yellow wires are swapped.

For anyone that hasnt been to England before the wall socket does not have an RJ11 or RJ45 port, its a "BT" (British Telecom) socket.

So I still needed to use a 4-wire RJ11 cable and splice it onto a 2-wire RJ11 to go from an FXS to my BT wireless phone. I guess I must of had a 4-wire RJ11 cable which was has different pin-outs to a simple straight through... I Thought al RJ!1s were straight through.

Barry, what cables do you use? Because I used the ones that came with my BT wireless phone and it didnt work, no dial tone and FXO card could not see dial tone either. To investigate what I did was cut an RJ11 2wire cable and plug the RJ11 side into the FXS, then touch the bare wires onto the outer 2 pins of the phone and got dial tone and ring.