John, go outside to Network Interface Device, (grey box on the side of the house called the NID), and open it. See how many lines are attached to the terminals. Chances are good that there have been many technicians in this box over the years and things may not be "straight" in the box. Be sure all lines leading into the house are connected, if not, attach them to the screwpost terminals, else go to Lowes and buy the "Scotch lock" crimp on connectors and join the wires. Pairs are red/green as the primary circuit with yellow/black as secondary. If any new wiring has been added to the system, it may have been done with blue/blue-white pairs and orange/orange-white paired wires. The blue/blue-white is the primary and blue goes to red, blue-white goes to green when joining to older exisiting pairs.
Chances are equally as good that the wires may have been pulled from individual jacks through out the house. Open and inspect them, reconnencting as needed. You could also pick up a Sperry test device from Lowe's that plugs into the jack and either glows green or red, depending on line status. Could make it simpler to determine the need to open the jacks.
All in all, it's not difficult work. Good luck.