Since the foreign call centers don't seem to care about the Do Not Call Registry, I created one new address book entry called "Spammer" to be used for all of them. Then whenever I get a call from a new number, I just add that number to the Spammer contact. I have that contact set to automatically send all calls to voicemail so I never even hear it ring. Once in a while I dump any voicemails they left (they almost never do) and purge any appearances of that particular contact name from my call log.
I may not be able to keep them from calling, but at least this way I don't even have to look at my phone when it rings to see who it is.
