Dialing 67 (or star 67) is also known as “Caller ID Blocking”, it allows you to block your number, on a call by call basis, from appearing on the caller identification (ID) unit of the person you are calling. There is no charge for using Caller ID Blocking (67), as there is no need to add this feature to your account; it is automatically available to you.
However, if you want to block caller ID from all calls permanently, you need to call your phone company and request per-line blocking to block caller ID from all calls made from that specific phone line; and the phone company may charge a fee for this service.
Of interest, there's a new iPhone app called The Call Block app that tries to make telemarketing calls less intrusive, calls from telemarketers can be set to only ring a soft beep that the user can then ignore. Find out more at Los Angeles Times.