I bought these:
https://www.telephonesonline.co.uk/b...l-blocker.html
You do need caller display enabled on your landline for them to work.
You put all valid phone numbers in the contacts list and they get through right away.
Any other numbers are asked to say who they are and can be rejected without speaking to them.
Automated calls can’t answer so get dropped automatically.
The low price is down to the products being ‘refurbished’ but in my experience are as good as new.