I have an intermittent fault on the line. Some days all is well then the connection keeps dropping out. I plugged the router into the test socket, still dropped out. I then dialled 17070, with a corded handset, also from the test socket and heard a persistent crackle. It is a long way to the exchange, around four miles.
My ISP has agreed to request a BT engineer but I wonder whether it will crackle whilst he is here as some days the line is OK.
Is it possible to buy any sort of device that would measure or record the elusive crackle? It would be cheaper than the call out.

No. Ive had the same problem with intermittent hissing and crackling, the openreach man comes out and its not here, what a joke.

Yes that is a rule of thumb or sods law whichever way you want to look at it.
If it is intermittent it will magically disappear just before he knocks on the door .
All you can do is plug into the test socket with a corded phone and see how it goes (no filter) ..... or do that when it starts crackling.

I had the sane thing years ago and the problem was that a previous tenant had put the cable from bt's little junction box to the master socket under some floor tiles they laid.
This was pointed out by BT (think before Openreach) and the guy said there would be a charge if I wanted to fix it . Luckily the engineer was one of us (whoever we are ) and put it down to an outside fault and free recommenration as to what to do.

It is ridiculous that BT do not have a procedure for intermittent faults. To simply blame the customer is wrong, it achieves nothing, except to create distrust.
We had some warm sunshine over the weekend and the crackle has disappeared, for the moment.
It is a long line, I don't expect it to be perfect but I do wish that BT were on my side.