Is it something to be concerned about

Some of the cold call scammers are able to highjack legitimate UK numbers, so the caller ID shows a UK number but in fact it's international. You can block that number, but the next time they call it will just be another highjacked number, so the blocking won't have worked. We look at caller ID and leave unknown ones to go to answerphone. It doesn't cure all of them and there's a risk of missing calls from big business who won't leave messages but in the main it works.