It can ring both lines but only if it is connected to both lines which can cause a slew of problems. When professionally wired red/green is line 1 and yellow/black is line 2. You can test the numbers by connecting each pair to the red green on the jack and plugging in a telephone to make a call to a cell phone. If that jack is going to be used for just your fax connect the correct phone number pair to your red/green on the jack and viola.