Best method is as below (I've copied it from a post I made on another forum), if you use your existing wiring then I suspect it's only normal phone wiring and it could be susceptible to interference.
If your capable of basic wiring you can run your own extension for the modem, I would not want any of BT's cables going any further than my master socket - BT own them and it is illegal to tamper with or alter them, this includes extensions they fit.
BT will fit a new master socket face plate, you can legally remove the lower section of the face plate, inside are two teminals marked A & B (see link below), you need to run CAT5E network cable from these two terminals to a RJ45 socket next to your modem. You need to use 1 pair of the wires in the cable, and connect these to pins 4 & 5 which is the blue/bule-white terminals in the RJ45 socket. The RJ11 modem cable will plug into this socket and your modem.
The CAT5E cable has 4 pairs of wires, so you can use another pair to run a telephone extension to where your modem is as well, all in the same cable
You will need a Krone punch down tool, I recommend this one http://www.clarity.it/xca...38&cat=257&page=1
and avoid running the cable with power cables as they can cause interference.
VDSL2 extension and socket http://phil.lavin.me.uk/2...-uk-fttc-vdsl2-explained/
Telephone wiring - http://www.rob-r.co.uk/other/UKphonecatwiring.htm
Parts and further guides http://www.clarity.it/
Cat5e cable http://www.clarity.it/xca...6214&cat=0&page=1
cheaper sources can be found.