Private, unknown and unwanted calls are an inevitable part of having a phone. Many times, the people behind these phone numbers are telemarketers or pranksters. Having a caller ID will enable you to see that the caller is hiding behind a blocked number, thus allowing you to reject the call before even picking up the phone. However, there are a couple of other ways to block private numbers altogether.
Do Not Call Registry
Sign up with the National Do Not Call Registry (see link in "Resources"). This puts you on a government-supervised list that prohibits telemarketers from calling you. Allow 31 days for the registration to take effect. If, after this time, you're still receiving telemarketing calls, contact the National Do Not Call Registry website and file a complaint against the company that called you.
Inbound Call Blocker
Purchase an inbound call blocker. This is a device that connects to your home phone and requests that all people calling you enter a special code before their call will be routed to your actual phone. If the caller doesn't have the code, you'll never even hear your phone ring. You must give this code to all friends, family members and co-workers, however; otherwise they'll be unable to get a hold of you by phone.
Anonymous Call Rejection
Sign up for an anonymous call rejection service with your phone company. After you receive a call from a private number, dial *60, and that number will automatically be blocked from calling you again. The next time they attempt to call you, they'll be routed to an automated message telling them that you're no longer accepting their calls.
Speak with your phone service provider about a service called Priority Ringing. This allows you to assign a certain number of phone numbers the ability to call your phone, while all others will be routed to voice mail. Assign the numbers to your closest friends and family members, and if other extended friends or family members call, you can call them back after they leave a message.