From my recent experience, it would seem by using Nokia's application,number can be blocked. By doing this, it also blocks SMS TEXT message even though this facility is not set in the Application set up. Blocking 'WITHHELD numbers also blocks TEXT message from that source. This latter situation has been bugging me for over a year and I have not been able to get a satisfactory solution from either of the Mobile Network operators I have been involved with. Both seem to point the finger at Nokia.
Usually your mobile provider/network will block numbers for you.
I have been advised that the Operator does not BLOCK calls it is down to the USER to select which calls to BLOCK. Which I would consider a more likely process. If the Operator blocks numbers then the distribution of calls would be effected.
I don't know how to block a number but I have found a solution - enter the number in your contacts and I give it the name rubbish or something similar. When that number rings and you see the name given, just close it. If your phone only accepts one name
just add a number to it ie., rubbish1, rubbish2 and so on.