I bought this lead to couple my dial out alarm system to a BT socket (the alarm system has a RJ11 socket). After a few hours of pining it nicely halfway around the house to skirting boards etc., I found it didn't work with the dial out on the alarm system. However, it worked perfectly with a telephone when plugged in. Closer inspection of original dial out lead on alarm system, showed 2 wires being used in a different configuration. I since have bought a normal BT socket to BT plug extension and used the original dial out lead supplied with alarm system to plug into this and it's OK.
So, the moral of this little episode is (unlike me) to check what you're buying!
If you want a lead to extend your fax or modem etc. it's the bees knees!