RJ11 Data Modem Cable

You need one made up using BT  CW1308 cable then. That’s what Open Reach use so meets all the relevant standards. 

It’s  copper, twisted pair. Use only one pair - convention is the blue pair. 

You can buy cables made up from various places - I’m not sure whether forum rules allow me to tell you where mine came from, but google it and look for one with loads of hyphens in the name. 

Thanks TALLGUY I spotted a Belkin earlier and ordered, i did see the BT one you mention but they were like 20m lengths 

That’s a pity - mine is either 1 or 2 metres long, so they used to be available in shorter lengths. 

Back in the day Maplins used to do a screened one - I never could work out how the screen was connected.

Belkin seem to make reliable cables so I’d think you’ll have no trouble.

Obsydian posted:

Thinking weakest link and GIGO, surely the throw away cables can be bettered at least with a shielded or twisted pair ...

We ate all spending effort on routers, switches, some fancy cables, but what about the wall socket to router cable ?

This is a good point.. albeit nothing to with data validity etc, but will affect xDSL synchronisation speedfrom your ISP potentially and near field emissionsrDiating from the cable. Best advice is keep as short as you can and twisted pair cable. Indeed CW1308 is a suitable cable specification and comes in 2, 3 , 4, 6, 10, 12 and 20 pair variants from several manufacturers... and is designed for telecommunication and telephony use.

http://www.farnell.com/datasheets/23025.pdf

I changed my freebee type flat cables (wall to UPS & UPS to router) for twisted pairs,  not sure the brand but not expensive.  That was in the days of ADSL2 & less than 20Mb/s line speed;   they made no real difference to down-link speed unless it was running a load, something like a web-video, when the new cables raised the previous 16Mb/s (or less) to close to the same as no load 20Mb/s speeds.