When I use the KX-T to dial on the line without a dial tone, I get CO error messages like "Your call did not go through", or "It is not necessary to dial a 1 when dialing this number". If I just leave it off hook without dialing anything, it says "Your call did not go through" rather than the usual "If you'd like to make a call...". So the CO line is seeing something, but not liking what it sees.
Obviously then the line IS going off hook. It sounds to me like either your system is seizing the line without your lifting a handset or there is something else on that line like an alarm dialer that is stuck off hook. Could also be bad wiring, a corroded jack, etc.
What happens when you call that line? Do you get a busy?
-Hal
