I want to connect my home phone line network to my ISP box and my ISP box directly to the France Telecom line using twisted pair cable.

Initially, room 1 was connected directly to the FT line, so I disconnected it in the FT box, connected the two wires coming from the room and the two wires from the FT line to the twisted pair cable. At the other end, I connected two phones socket, where I plugged the ISP box.

Now it looks like this:

home phone line

The problem is that since I did that, my noise margin dropped from 7,2dB to 5dB, CRC errors started to appear, and the download bandwidth dropped from 7,8Mbps to 5,8Mbps. If I disconnect the phone from the ISP box, it's back to normal. Also, using a phone filter on the phone socket connected to the ISP box improves the bandwidth up to 6,8Mbps.

I think there's some interferences between the phone and the ADSL line. How do I solve that?

I'm using cat6a FTP cable, do I need to connect the shielding to the ground?

Do the two wires from the phone line have to be connected on the same pair or separate pairs? Is their a norm on connecting those? (like ADSL on blue/whiteblue wires and phone on green/whitegreen wires for example)

Would I need to have two separate cables for the ADSL and phone?

  • Why not leave the box where it was and just run Ethernet from room 1 to room 3?
    – longneck
    Jun 15, 2015 at 19:54
  • Because I want the blue phone network (local, connected to the box which has a VoIP service) to be separated from the red one (external one, from which ADSL arrives), and also because my Ethernet network is centralized in room 3 (I've a switch in which all the other rooms are connected).
    – piernov
    Jun 15, 2015 at 21:20

1 Answer 1


After some testing I finally found the answer myself.

I connected the cable's shielding to ground and I connected the wires this way:

(phone cable) - (cat6 cable)
  White ADSL  - Blue
   Gray ADSL  - WhiteBlue
  White Phone - Orange
   Gray Phone - WhiteOrange

(colors are for french phone cables, they are different in other countries)

I soldered they directly. Maybe it would be better to use a RJ45 connector.

Now my modem is synchronized at 8,5 Mb/s. Still some CRC errors, but it's not a big problem since I can get something like 7,1Mb/s when downloading.

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