In making a case concerning the use of TFD's, police must recognize the device, establish possession, and be aware of circumstances that indicate the device is being used to defraud. While TFD's may resemble automatic telephone dialers, they may be distinguished by their dial tone and by their inability to complete local calls. There are three main types of TFD's used to evade payment of long distance charges. 'Blue boxes' originate fraudulent long distance calls from regular or pay phones. They often resemble a calculator and may function as one and have 12 to 15 buttons used to activate tones that secure an open long distance line. 'Red boxes' are used exclusively with single-slot, coin-operated telephones by duplicating the tones of coins dropped to pay for a call. 'Black boxes' are devices attached to a telephone line that enables callers to place free long distance calls to the phone to which the device is attached. 8 footnotes.