Because the wires are not twisted pairs it was never intended to support two POTS lines. Problem is crosstalk. That's not to say that it was never used for two lines, you can get away with it but the longer the cable the better the possibility you can have crosstalk problems. (And don't ask me how long because I have no idea.) I remember that you will hear the ringing from one line while the other is in use.

-Hal


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