With Caller ID, customers who pay a monthly fee and purchase a display device can see the number and name of the person calling before picking up the phone. Some people believe Caller ID will help reduce harassing or unwelcome calls. Others, however, raise privacy concerns about the technology since subscribers to the service can capture callers' phone numbers without their consent.

To help consumers protect the privacy of their phone numbers, state public utilities regulators (for example, the California Public Utilities Commission) require local phone companies to offer number blocking options to their customers.

There are two blocking options to choose from. If the customer chooses Per Line Blocking (called Complete Blocking in California), their phone number will automatically be blocked for each call made from that number. If the customer chooses Per Call Blocking (called Selective Blocking in California), the phone number is sent to the party being called unless *67 is entered before the number is dialed. When the number is blocked by either of these methods, the Caller ID subscriber sees the word "private" or "anonymous" on the Caller ID display device.

Because of these blocking options, Caller ID is not likely to allow you to capture the phone number of the determined harasser. Most harassers will block their phone numbers or will call from payphones. However, Caller ID can be used by people receiving harassing calls to decide whether to answer. They can choose not to pick up calls marked "private" or numbers they don't recognize.

A companion service to Caller ID, called Anonymous Call Rejection (ACR), requires an incoming call from a blocked number to be unblocked before the call will ring through. Use of this feature forces the harasser to disclose the number - by entering *82 - or to choose to not complete the call. But a determined harasser can get around this feature by using a payphone. This service can be added to a consumer's local phone service for a fee or at no charge depending on the carrier. It is activated and deactivated with the touchtone code *77.