I never answer my landland unless I know the caller. I'm a miserable git that way.
Besides, the caller can easily leave a message.

Same applies to my cell phone and not many folks get that number from me.

I do have a recurring issue with an international call that's actually a fax m/c, they never give up until my answer phone is full.
BT don't have any solution.

I'm sure the callers are just doing what they get paid for though