I was wondering how simple it was to rewire a phone jack for 2 lines. The phone co. charges $100 to do it. Can anyone help?

1. At your network interface box (usually mounted outside; it's where the phone company's lines come to your house) look for a bundle of wires from which two wires are wired to posts. This is your line 1. Generally, it should be red and green, but it really doesn't matter.
2. Now from this bundle, pick two other colored wires (generally, black and yellow are used for line 2) and wire them to the jacks for line 2 in the network interface box.
3. For each outlet that you want wired for both lines, open it and wire the same two colored wires that you just wired up for line 2 to the other two posts within the outlet. Most home phone jacks have 4 posts for 2 lines - but if yours has more, then look for which ones are marked for line 2.

Here's a more detailed step by step article:
http://www.homephonewiring.com/add-line2.html