Telemarketers are people who make unsolicited phone calls to you attempting to sell something. Telemarketing companies are capable of dialing hundreds of phone numbers in a short period of time.
Put a message on your answering machine indicating that you do not accept unsolicited telemarketing calls and to please remove you from their list. This method may have been more useful in the past because telemarketing calls are placed by a computer. Once your answering machine picks up, the call is immediately terminated and your number is placed back in rotation. You should receive fewer calls from charities.
Telemarketing calls are computer generated by the hundreds. The call is connected through to a live agent only if you answer the phone. If you answer after your answer machine is activated, you can avoid speaking with a telemarketer. This method may not stop telemarketers from calling, but you won't have to speak to them.
Telephone devices that stop telemarketing calls work by sending a signal to the automated dialer indicating that your phone is disconnected. The device is activated after your answering machine goes on, but you don't need to leave a special message on the machine.