ufff, we had been able to block enough numbers anyway. But as a small company, there are not so many numbers to block. (The tax office and the value-added taxes will still come along at some point ;-) )
It was more important to us that a name or designation was displayed for the number. This way, you could often recognise the call centre right away.
I tried blocking the numbers on my mobile phone, but the call centres are not stupid and change their numbers regularly.
By the way, it's still in operation. We had to change the accu-batteries after more than 4 years. But they are type AAA and therefore cheap to get.