Re: "Is there a pre-filtering device that will sit between your phone and the wall socket"
well, I've been thinking much about something like this!
A device that counts say 10 'rings' of an incoming call before the bell (or other sounder) rings out appears to be a simple and good filter for these pests.
Now, not wishing to breach the rules about attaching non-approved electrical/electronic equipment directly to the Phone line, I'm trying to develop a 'ring tone' detector which uses the ring tone 25Hz current in the wires with some sort of 'clip-on' sensor.
Anyway, if these calls are answered, the telco collects it's fee and I really think they don't deserve this at all for permitting these calls in the first place.
+1 for all International call blocking as well as withheld numbers