Re: You mean, like, Caller ID?
I just whitelist.
Unless you present to me a known CLI that I have chosen to accept in the past, nothing gets through. You can't contact me? Shame. Maybe that's because I never gave you my number in the first place (or didn't want to but was forced to, or someone else gave it to you, etc.).
For personal use, I just mute the default ringtone and make known contacts have a ringtone that sounds.
For business use, I only bother with SIP and then it's just a matter of pushing them through a SIP-based menu if you don't recognise their number from your whitelist. They don't even connect to my site unless they can get past that menu (which has an option to leave me a voicemail, which I treat just like unknown spam email).
If you're hiding your CLI, I don't want to talk to you (the old joke: You want to hide your number, fine I'll 100% respect your privacy by not taking your call...). If you need to contact me, you'll be given an option to leave a voicemail. If you can only get through by calling from a proper CLI-number... oh what a shame... that's like only being able to send me an email if you come from a proper domain, with proper reverse DNS, that's not on a blacklist, etc. etc. etc. No different. At worst case, I google the CLI phone number and if nothing comes up, I assume you're a spammer.
And if it's anything even vaguely important, like an unpaid bill, a contract you need me to sign, or anything along those lines... you'll send me a letter. And I'll discuss it with you at my convenience.
I've cut the landline. I have a 4G box instead. The number of that is my "spam" number because... well... who cares, it can't even ring anyway. My mobile phone (20+ year old number) operates a whitelist like above. I get less spam on that than anyone else I know. My inconvenience factor of "someone couldn't get hold of me" is basically zero - people I know have no problem getting through. And my backup plan is that I have a Draytel SIP account in the background, connected to my router. If I need to, I'll move to that, and send you through a verification switchboard menu first.
Fact is, the last 10 years I've had that option and I've not been pushed to it yet.
I'm on the TPS, I don't sign up with my numbers to anything, and I get almost nothing anyway - maybe a number that people online say is a "have you had an accident" caller once every three months or so. I'm presuming they just try every possible 10-digit code or whatever and hang the TPS. They don't get through. 99% of the time they don't even get to a voicemail. I don't have the time to mess with them or play games or even answer them so I don't... if my phone is ringing from an unknown caller and I need to use the phone, I just press hang-up and dial the number I need.
I've not yet missed anything important in 10+ years. I don't see why others would suffer the way they do. A friend of mine just bought one of those services/phones that blocks so-many-CLIs and things... it's useless, and the blacklist fills up very quickly, and they have to PAY for it... to the same people who are delivering those calls and being paid to do so in the first place. Sod that.
If it ever really becomes an issue for me, I'm only about 2 days from getting a bunch of 07 numbers from a SIP provider, or something like Skype, and just let them get spammed to oblivion and never answer them to anyone I don't know. Cheaper all round. To be honest, I think I'd just do it through my router or a PC with a load of 3G dongles... and just load Asterisk, FreePBX or similar to send them in circles if they are unknown. It's not yet come to that, though.
Why a telco thinks they'll see anything of my money while this is happening and they aren't taking the simplest of actions against it, though, i can't fathom.