Many homes were wired with a "daisy chain" or loop. The Blue/Blue-White lines are probably going off in two directions, no way of telling from your description, but it could be one to the demarc and one to another jack. The Orange/Orange-White pair could also be going to one of the first two locations, or to a third location. There may even be yet another Orange pair that isn't connected that could be used for line two. You need to figure out what is available for use, if you don't want to change any of the other phones. You can do this by trial and error or by "toning" the lines. (Ask your Home Depot electrical guy if you don't have one of these.)
You could also have a situation where the lines are "home run" but someone added a jack by extending a line from that jack to another.