Blocking the number won't have any effect as it shows a different number each time they call.
I've received a few recently and the numbers do not correspjnd to where the caller is ringing from - always try and google the number as I'm speaking to the person who calls me!
Had one last week, allegedly from talktalk (who I am with) saying there was a problem with my system sending out messages ... but the number showing on my phone was a Barnstaple STD code, so I asked the caller what the weather was like in Devon! He couldn't understand why I was asking, so I persisted as he was trying to do his spiel. He obviously wasn't in Devon so I ask where he was, and he told me London.
So, I asked him to explain how it was that my phone was displaying a Devon phone number.
His comment was that there must be something wrong with my phone. I replied saying there must be something wrong with his as it has just dialled a number which was on the Telephone Preference Service list.
He hung up!