Something else I've done to discourage telemarketers, etc is to ask them to hold a moment while I set up the digital recording device to record our conversation. Than, .....
"Hello. You still there? I've got the recorder working now. Please start over, and I'll have a few quick questions for you to answer."
Than I'll interrupt and require them to identify themselves, their phone number, company, employee ID, and ask questions they do not want to answer. (What is your residential address? Phone number? Date of birth? Social Security number? Driver's license number? Mother's maiden name? Marital status? Employers occupational license number? The list can be endless, and they always terminate the conversation.)
In addition to being on the "Do Not Call" registry, I must be on another list somewhere, (that those companies pass around among themselves) because I do not recall receiving an unsolicited call in a year or more.