Fed up with robot phone calls saying my broadband connection will be terminated tomorrow unless I ............at which point I hang up. Oddly enough I still have my broadband connection!
I don't understand why BT call protect is not working for you. It always works for me. You can block the calls in your account on the internet. These nuisance callers do use more than one number, so may need blocking more than once. I have not had any calls like this lately, but vaguely remember something on those lines in the past.
Due to these, and other unwanted calls, I registered with Telephone Preference Services, and have had none of these calls for well over two years. Registering is Free and only takes a couple of minutes.
I used a similar service for stopping unwanted post. (letter box post).
The problem with withheld numbers is that some of them are genuine, especially those from organisations with switchboards like hospitals so I tend to allow them. Only very occasionally will a rogue call slip through. All international and unavailable numbers are blocked.