It works the same way that other Ugly Mugs schemes do, the Ireland one being a fairly well known one. It's not legal to supply people with identifying personal details which would include a phone number (and is also the reason we can't publish them here), so nobody can legally post up pages of phone numbers for random interested parties to leaf through.
What you can do in the event that you think you have the number of somebody dodgy is type it in and see if there's a hit. You still won't be given a full number, just told what the report is that it's linked to - you have to have the number in the first place .
The difference between here and there is that here you have chance to read the story and see the signs to watch out for, or work out what sort of risk the guy is/is the report legit.
I had a guy phone me a few months ago who's number came up on UM, he sounded fine on the phone, wasn't doing or saying anything to set alarm bells off, and all UM said was "this guy is in our database".
I couldn't see him anyway, but did spend ages searching here and reading through the last 12 months of reports on the NUM site, couldn't find any detail at all.
So guess you just have to trust that it is a genuine report and someone to be avoided - sorry if that sounds daft, I do know of malicious reports on forums and between other providers - myself and two regulars have been on the sharp end of that twice, although that wasn't saafe but could have been.