Even if they do leave a message, it can be a telemarketer. In fact, the scammers I mentioned in my post above would leave messages. They are able to do it before the blocking of their number takes effect on my phone, too.

These telemarketers (most are the scammer variety) get worse all the time:

Block them, and they still bombard you with calls from multiple numbers filling up your phone's block list.

and...

They spoof their calls to appear to be from your area code, or, as I posted above, they appear to even be YOU calling yourself.

Obviously, the worst of them do not adhere to the "Do Not Call Registry."

Many of these telemarketers call to the point of harassment. Hopefully, some day there will be a way to bring them to task legally for their actions, but for now they seem to be getting away with it.

The irritating thing about it spoofing other numbers? I've had a couple of people call me who had my number spoofed and didn't answer, who thought it was me calling them.

My rule is if I don't recognize the number, I either ignore it or answer and immediately hang up. ie, click on, click off. If it's important, I figure they'll call back. No skin off my nose if they don't. 

(Due to cable bundling, we have a land line. 90% of the calls we get on it are bs. The cell....only about 15 to 20%.)

I got a call once from a company offering me preferential rates on pallets. It was the umpteenth call that week. I could not help but lose my temper and shout, "I live in a flat, I'm not a business!"

I get fewer scam calls on my mobile phone, but one is still one too many.

I've had the 'Microsoft representative' call. On one occasion, I replied, "Not sure how I have a virus as I own a Google Chromebook - and they don't get viruses!'

I can't say for certain about the exact law, U.S. or, in your case, the U.K., but it could leave you open to some sort of lawsuit. I must confess, I have been tempted to use something like an air horn on a telemarketer, but even as scummy as they are, I'd hate to cost someone their hearing.

EDIT: As ever, I am referring to fraudulent individuals or individuals who misrepresent where they are from, not people sincerely trying to sell breakdown recovery insurance or whatever (they are simply doing a job, unwelcome though it is).