Phone call blockers are relatively simple devices that work on the same principle as caller ID. When we make a phone call, our phone number is transmitted along the phone line as a series of tones. Call blockers recognize any numbers you've listed as prohibited, intercept the calls before your phone rings and play a message notifying the callers that they've been blocked. Some call blockers also give you the option of requiring unknown callers to identify themselves before your phone rings.
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