How to restrain telemarketers?
Pick up the phone when it rings and let the other person start talking.
After few seconds, interrupt and ask the person for his/her name and the company name they work for. Once the caller is identified they become more careful with their actions.
Tell them that you are not interested in the stuff they are promoting. It makes them understand that it is an unwanted call.
Politely but firmly ask them to delete your phone number from their lists and to not call this number again.
This should work in most cases. Telemarketers will accept your request because their goal is to sell not to upset people. (Read article)