Probably happens because 201 is the only station in the 500 group.So this only started when you added the 4COP card? What ring voltage and frequency do you use there?And a co line is 48v when on hook? I still wonder if that card is compatible with your lines? I would be interested to see if you put DT XXX keys for the lines on the phone to see what happens. When you answer that 500 call you said you get silence ,what does the display on the phone show?