So just tried customer services to get all 0843 numbers blocked and the operator was like something out of Little Britian: 'No we cant block incoming numbers' said seven times before I got bored and hung up before being rude.
Why not just brief the teams to say: 'We cant do it from here but there are some applications available that appear to help. Please look at U-switch or your app store.'
At least that's helpful !
I use TrueCaller and find it very good at filtering out spam and other unwanted calls.....