Re: The Landlane 'Tax'....
"--So why can't we always hang-up on callers, anyone know why this 'feature' exists?"
It's to allow someone to put down the phone where they've answered it and go and pick it up in another room. There's a timer that starts running when the phone is put down and if one gets picked up again before the timer expires the call is still active. That timer length has been shortened recently as AndrueC mentions above.
"How about adding a calling list filter, similar to Skype etc. So that nuisance calls (including silent calls) can be automatically screened out, especially for vulnerable older generations"
How do you know what's a nuisance call in advance to know to filter it out? The criminals often spoof the calling line details in countries with lax regulation to make the number appear to be a valid UK one. See the post office analogy above - blocking these calls is like asking the post office to confirm the address of a letter's sender before they put it through your letterbox.