Abstract: The device is inserted at the jack between the tip-ring signals of a telephone company's line and the customer's telephone lines and equipment to isolate the location of telephone problems. A first tri-color light emitting diode shows either "off" or "green" when a problem is detected, "red" if a telephone company line reversal exists, or "yellow" upon detection of a ring generator signal. When problems are detected, a switch is manually positioned to connect a second tri-color LED between the tip and ring signals while disconnecting the first tri-color LED and the telephone equipment. If the second tri-color LED is "off" then there is no dial tone; if it is "green" then the tip-ring signal lines are reversed; if it is "red" then the problem is within customer-owned telephone equipment. A battery test of the tri-color LED's is optionally incorporated. A variant test device connected at a RJ21 jack for testing ground start telephone trunk lines is also disclosed.