I have a little fun with them. I have VOIPO, inexpensive, but it lets me block several ways. Since they won't stop, I just forward their number right back at them and in some cases it causes their system to go into a Loop. Cool.
See SEOscam.net. Voipo at FlatRateCalling.net
The company is called LeadsMatic under the parent company Mobile Telecommunications Ltd. They say they're based out of Seattle but it is a boosted member from India. Since they won't put me on their Do Not Call list after weeks of requesting it, I just call them back and mess around with them to waste their time.