Why not do this?
Wouldn't cost a lot, if anything, networks do have the functionality/ability to do so, all it needs is a front end so that customers could selectively block calls if they wanted to, per account.
- unknown callers, (unknown/private number caller id)
- international calls, (calls from outside the UK)
- editable list of individual numbers with wildcard so people could block 084* to block all 0845 and 0844 numbers
Would be the first network to offer the service so a usp, and many customers would benefit as it would reduce spam.
Only drawback is that it would potentially reduce revenue collected from termination charges, but I'm sure you could still get that by redirecting them to a recorded message telling them the caller has decided to reject the call or something similar.