I have a 2-line telephone that shows when a line is in-use (i.e. if an extension is picked-up, the indicator will light up on the phone). The two lines are being supplied via 1 RJ-11 cable (1 line on each pair). Both lines are supplied via the wiring throughout the house but here's the issue: - The lines DO NOT show as IN-USE when both phone lines are connected to the telephone

When 1 line is connected to the phone, the indicator lights up when an extension is picked up. I've tried both lines individually and the indicator seems to work fine. The issue only seems to happen when both lines are connected. I suspect the voltage is not dropping (indicating an off-hook) but does anyone have any suggestions on where to begin troubleshooting? Thanks.

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    \$\begingroup\$ It seems odd that the two lines would have an affect on each other, but to test your theory, you should measure the on-hook and off-hook voltages of both lines in all combinations with a voltmeter. \$\endgroup\$ – tcrosley Apr 30 '14 at 3:26
  • \$\begingroup\$ If you supply a part number and user manual reference for the two line phone we could perhaps have more chance of troubleshooting for you. I am not sure how you have things connected and what your tests reveal. If you are connecting the two phone lines in parallel all bets are off as this should never be done. \$\endgroup\$ – KalleMP Aug 25 '17 at 19:29

I would investigate whether both lines have the same polarity. This used to be important, but modern phones don't seem to care, so installers connect house wiring to the phone lines without observing tip/ring polarity. It may be that one of your lines has tip/ring reversed, and your indicator circuits can't handle that.

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