To expand on RobertoS's answer, on SKY it is also 17070; but Option 4.
As usual with that type of service, a list of Options are read out; and you select whichever is appropriate.
Some Options on the BT version are effectively "password protecred"; but the QLT is not protected.
I have it in my phone's directory, although I remember the number readily enough.
In odd moments about once per week, I carry out that first level test as RobertoS has described - only ifI hear something untoward, would I resort to using the Test Socket and working back.
Edited by deleted (Thu 16-Jan-14 19:20:29)