It might be counter productive for telcos to block them, as it’s apparently quite easy to forge the number the call is from.
This is “as I understand it” and could be wildly wrong, but:
There’s two caller ID idents on a call. The CLI you see is a “presentation” number, it’s easy to withhold (with the prefix 141 in the UK) and trivial to spoof. Surprisingly perhaps, it’s not illegal in and of itself (so long as you own the number you’re spoofing)- corporates do this all the time, for example to switch between having employees’ DDI numbers presententing their DDI or the main switchboard number. Point of note here is that when you withhold a number it still gets sent, only with a flag set to say “do not display.” It’s down to the receiving end to honour this flag – this is why if where to dial say the emergency services they’ll still see your number if you tried to withhold it, they have special equipment which ignores the flag. (Either that or the next paragraph applies here anyway, I’m not 100% certain.)
There’s also an “engineering” CLI. This is always sent, and AFAIK much harder to spoof. So it’s simple to forge CLIs at an end user level but the telcos should still be able to see the real number.
I think, anyway. I’ll see if I can get clarification from a higher authority, my boss is an old-school phreaker and may know more than I do.