Re: What I don't understand
"I wonder though if it couldn't be set up to have a whitelist that can directly call, and other numbers get an answerphone which can be looked at "off-line". That'd solve my above problem, and still avoid having to deal with the so-and-sos."
That's more or less what I have set up with various rules on my phones (which are all VoIP now, and fed into 3CX; even the line that carries the DSL is bridged into that, via a FritzBox).
Office number, all calls come through, unless explicitly blacklisted, which are dropped, and withheld/unavailable, which go straight to voicemail.
On the number in the phone book, all calls unless blacklisted get an announcement reminding them I'll be very rude if it's a sales or survey call, and they can press a button to speak to me, or another to leave a message.
On the ex directory number, whitelisted numbers get through directly (typically older/more bewildered/best loved members of the family), blacklisted ones get dropped, and the rest again get the choice of message or reaching me
Net result is no cold callers that actually bother me, with a few exceptions on the business line, and tradesmen etc can still reach me, as long as they listen to the instructions and press the right number.
There are a few who call and start with "Can I speak to Mr Nigel Whitfield?" to which my usual response is "Who are you and what do you want?" If they don't give a good account of themselves, they get told to go away pretty sharpish.
If the bank calls (First Direct), I won't answer security questions on a call that I receive. I ask for their extension number and department, and call them back.