I got signed up with the real one years ago. It mostly works, but you still get the odd one blundering through. Like last weeks'. The phone rang, I answered, it might well have been someone that I wouldn't have minded speaking to, silence for a few seconds. Click, NU tone. Dial 1471, either "number withheld" or the rather strange "There is a semantic error" in a transatlantic accent. This several times a day for about a week. Eventually, a voice, someone there, asking me if I wanted any of their superb financial services. I asked if he was a part of the halfwit company who had been calling anonymously for the past several days, and if they thought I would be impressed by such a sloppy, ham fisted sales approach, and if they thought that I would be impressed by that approach, they had just mortally insulted me (Words used may have differed). Click. 1471. "Number withheld". Touch wood, been quiet since.
It seems that an automated machine rings numbers from a list. No reply, move on. If the call is answered, the machine calls a centre where someone might or might not answer. I believe that this kind of operation is actually illegal in the UK, being basically a nuisance call, so the only companies operating it will not be bound by any UK or EU legislation, not only for their approach, but also for the reliability or security of their product.
You can have legitimate "number withheld" calls, but that kind of commercial policy can only safely be regarded as a prelude to theft at some level. Back in monopoly days, BT had a department and/or a procedure for dealing with nuisance calls involving tracing and an agreement to support a prosecution, not always effective because of the technology back then. Now there is the technology, but I suspect that economies due to competition have ensured that such a department either no longer exists or access has been rendered so obscure as to be. effectively non-existent.