How Blocking Incoming Calls Works

People with landlines have many options for blocking unwanted phone calls. Phone companies now offer their customers a variety of services such as call blocking, anonymous call rejection and priority ringing to help them deal with unwanted calls. Electronics stores also carry devices that can be attached to phones and that perform similar functions. Many software downloads available on the Internet claim to do the job as well.

The easiest way to find out what services your particular phone company offers is to call them or check the Web site. AT&T's site, for example, lists several ways to block unwanted calls. One of the most straightforward services is call blocking (telephone code *60); subscribers simply specify the numbers they want blocked, and when those numbers call, the phone doesn't ring. Callers receive a message telling them that their call has been blocked.

Other options include anonymous call blocking, which blocks any incoming unidentified number, personalized ring, which identifies preferred callers with a unique ring, and Privacy Manager, which prompts unidentified callers to identify themselves before the phone rings so you can choose whether to answer. These same services are common to many phone companies, although sometimes by slightly different names, and they typically come with a monthly fee attached.

If you'd rather not pay more money to the phone company, external devices exist that perform many of the same functions described above. Inbound phone call blockers, for instance, only allow callers with a special code to get through to your number. You would only give this code to people you actually wanted to talk to. Of course, with this feature, that random hottie next door wouldn't be able to look you up in the book and reach you. Call-screening phone accessories like these are often sold in electronics stores and online by companies such as Privacy Corps and Digitone.

If you want to go the freebie route, you can download free software that claims to block unwanted calls. This software works by hooking up your phone to your computer and letting the software screen incoming calls. Some of these downloads require you to have other hardware though.

Your options for silencing unwanted cell phone rings are more limited. Depending on the particular phone and service you use, call blocking may not be possible. However, some wireless providers do offer possibilities. For example, AT&T's Smart Limits for Wireless program, though designed to set boundaries for young cell phone users, allows you to designate blocked numbers that won't be able to call in. Smart phones also offer ways to block incoming calls. Smart phone call blockers include free or for-purchase downloadable software that you can get online and in stores.

You're probably wondering how your phone can recognize the calls you want to block. Can it sense salespeople on the other end of the line? Find out on the next page.

Blocking incoming calls isn't that difficult. It's an extension of caller ID, which displays the calling party's telephone number directly on your phone or on a device attached to your phone. You can read more about caller recognition in How does Caller ID work?

Just as your fingerprint acts as a unique identifier, so does your phone number. Every time you place a phone call, the number you are calling from is transmitted along the phone line as a series of beeps at different frequencies. Even when your caller ID box reads "blocked," the number was still transmitted but was hidden from your view by the phone company at the caller's request. Many people subscribe to a service that prevents their number from displaying; you can also dial *67 to block your number from showing on certain calls.

How Telephones Work will tell you all you want to know about sending information over phone lines. Without getting into all the technical details that it does, a phone call blocker works by recognizing those numbers that you've designated as prohibited, and instead of ringing your phone, it plays a message telling the person they've been blocked. Some phone call blockers can be programmed to reject calls from the last person who called. This way, even if you don't know the person's phone number you can still block unknown phone calls. With other screening devices, people with blocked numbers may be required to identify themselves if they want to reach you.

Unfortunately, phone call blockers aren't foolproof. Persistent callers can often outwit these technologies. For example, phone spoofing, as it is sometimes called, alters the number that is transmitted as the call's origin so the caller won't be blocked. Placing a call with a calling card, using a pay phone or using VoIP can also evade blockers. If you receive threatening phone calls, you should contact the police.

If you think about it, the prevalence of call screening devices is somewhat ironic. Telephones were invented to improve communication channels and enable people to connect to one another over long distances. Now there's a growing market for ways to shut down that channel and prevent people from connecting. What would Alexander Graham Bell think?

For more on blocking incoming phone calls, investigate some of the links found on the next page.