It does'nt know what line to use, whatever you puch down to the other pair is what you will have at the other end. You could pass one of the single line ports through the extra pair or another digital station or plain old dial tone. The only thing I have used it for was in a warehouse situation where I had to run molding across the floor for a desk out in the open and the customer decided to have another phone at his desk so instead of running another cable along the floor I just used the extra pair to run another set and plugged it right into the existing one.
Remember the little off white 267C line1/line2 adapter, Now Imagine that jack in the botton of the phone as the line2 hole. That is all it is It Just brings the 2nd pair to the phone. No electronics involved