a new line with a dedicated number or a second jack from the existing line?
If a new number and line, you need the phone company.
If a new jack from the original line with the same number then just get a line splitter from Radio Shack and your set.
This is what I have done and its works great, has been for over 5 years. I have my dedicated line comming in then I installed a splitter jack. This jack has four outlets for 4 different appliances off that one line. My phone and my modem are off that one jack.
However, you never want to overload that splitter. You will have phone problems and signal dilutes down.