Further to Strange_Visitor's reply, there are also call filtering applications for smartphones that are offered free of charge. I'd used one once, though I can't remember the name, a search on google should bring it up for you.
o2 is not able to block numbers. who said they can?
I did, and I was mistaken :robotsurprised:
What I meant was, in theory, only a provider can actually block a call (prevent it from ever reaching your phone).
The software that phones use can only auto-reject calls when they are received. Depending on the quality of the phone / software, the reject should kick in before the phone actually rings, but you'll still get notified that you have missed calls, so it's not a perfect solution.