In my experience, the best way to block telemarketers, fax calls, etc is to use a whitelist and a simple voice-mail setup. I don't do this within LMCE at the moment but basically every caller with an unknown/private callerid is prompted to 'press 1 to ring our phone' or 'press 2 to leave a message' in extensions.conf. I populate my whitelist table in msql from my address book so that if the number is known, it goes straight through and rings my extensions.