Often these no number callers are from outside the UK, callers from within the UK are, I think I am right in saying, now required by law to show their number. As with most laws in the UK the whole thing is a bit of a fudge, calls are not allowed from companies trying to sell you things, but if it is classed as market research, then it is allowed. All that said, if you do get a number, it is well worth running it through the WhoCallsMe site.