if you have call blocking set up,by dialing 14258
you enter your pin number to manage the list and then add the number that you want to block,or if you don,t know the full number you need to dial 14258 to block the last number when they next call
Is it not easier to note the calling number before answering the call and not do so if it is 0843. Or lift the receiver then drop it, which I do for "unavailable" incoming calls.