Seriously. I hate it. It's such an intrusion. You'll be in the middle of doing something, and off it goes.
It's always somebody selling windows or conducting research, but most of the time it's MIL.
Her record so far is 4 times in the space of half an hour. (Each time I've answered by the way, so not like she's ringing back as no-one's answered!)
The kids go to bed between 7 and 8. She KNOWS it's not a good time to ring, but without fail she'll ring at something like 7.45pm right in the middle of bathtime.
She knows she's not going to get an answer. So shouldn't really moan if no-one answers. 
WIBU to turn the landline off every now and again when we want some peace? (We still have our mobiles turned on, we're not completely uncontactable!)