Representative Expression

I have seen a lot of comments from people above saying "how did they get my number?" and "why do they do it?"
Let me explain...

A company based in Bulgaria for example, has a a few people or a bunch of computers that basically start off dialling +44 0000 000000 and work their way up to +44 9999 999999. (it's not quite that crude, but you see my point. i.e. they didn't get your number from anywhere, it was just the next one in the sequence of numbers they called).
So they didn't get your number from anywhere, you just happened to be the next in the queue.

The system dials the number, let's it ring once, and hangs off.

Why? There are two possible reasons.

1. Any number that rings is a valid number and could be added to a database of UK numbers to cold call at a later date.
2. And this is the most likely answer, and is becoming a major scam. Assume they call 100,000 numbers.
   It costs them nothing to ring a number and put it down before someone answers.
   It's free to them. But the number they provide as their caller ID is actually a premium rate line.
   If just 1000 people (i.e. 1%) of the people they call hit re-dial, that is 1000 calls to a premium rate number.  
   

Remember, it doesn't cost you a penny to dial a number, you only start getting charged the second the other party picks up the phone at the other end.
If no one ever picks up, you don't get charged.
It doesn't matter if you dial it and the line drops the call at the other end immediately, it's too late. Once you have connected to the line the other end, you've been charged the minimum call (usually quite a few pounds). At their end, the system is designed to answer the call and drop it immediately. They don't want to talk to you. They just wanted you to call back.

Assuming it was say £1.00 a minute, for just 1% of people they call returning their call, they just made £1000. And it cost them nothing to earn that.
But they don't just call 100,000 people. Their computer system can call millions of people. And if just 1% of people hit redial, that is an awful lot of money they've just made.
And with most overseas premium lines, it isn't £1.00.
If you did dial the number, go have a look at your next bill.

So unless you are expecting a call from overseas, and unless you recognise the number, never ever dial them back. If someone wants to speak to you, they would always call you back. Go look up the country code. If you are not expecting a call from that country, forget it.

Every time you ring back one of these numbers, you become part of the problem. You are contributing to their profits giving them the excuse to carry on doing what they are doing. It's easy money for them.

It is also incredibly difficult to have them shut down, since they don't usually do this from within the UK or the EU. The caller ID they are supplying can be easily spoofed.
Even though it says "03598815400011", you could be being called from next door. "03598815400011" is just the number they want you to dial back and is not necessarily the number you were called from.