We use CPR CallBlocker. It hooks into the line and has 400 pre-programmed numbers that it blocks. The call comes in, we check the caller ID, and if it's no one we know, block goes the button. I love it.
I have the BT8500 advance
You call it and have to announce who I are. BT decides whether to block the call or allow through. U can set to allow through but only leave a message.
If in the house when it rings and you don't want to accept u press 2 to block 3 to send to answer phone and 1 to accept
A god send
My neighbour who was scammed out of a vast amount of money now has a system that only allows 'trusted' callers. I think it's called TrueCall. She didn't even have dementia when it all started - just naive and far too trusting of some evil b*tch who said she worked for NatWest and was calling to tell her she'd been selected for a £1m lottery win...