Yes I know the post is close to 2 years old. My comments are for future readers.
*67 does not work when calling toll free numbers or government number. It generally works for other numbers. However, if they have a sophisticated enough phone system, then the *67 does not work. I have used the prefix before for some telemarketers and they could still see my number.
*67 is a good precaution, but it does not guarantee your anonymity.
Received a call from SSI a few minutes ago. They did not leave a message on my answering machine. They were conducting surveys per the message you reach when calling the number back...which I did after dialing the *67 prefix.
Sorry for the delay in replying. When calling a non toll-free number, *67 prevents your telephone number from appearing on a Caller ID display or some similar arrangement found on a predictive dialer. *67 does not work if you call a toll-free number (800) (888) (877) (866) (855) or (844). This particular number is a regular one so *67 works.