My landline is also set to ring only if the number is in the contacts list. The phone is a Panasonic and it has this feature as "night mode", I've just set the "night" hours to be all day. I now get very few calls from unrecognised numbers, I think after a certain number of attempts I drop off the spammers' lists.
We just don't answer the land line at all. Like Clipper, if we get a call on our mobiles that we recognise then we answer, otherwise we let it go to voicemail.
