There are two possibilities I’d look at:
1) Most builders use CAT5E for phone lines, so there is a better than average chance that that some of the cables are terminated on a phone jack wall plate. Any extra non terminated lines are going to be a pain in the butt to locate.
2) If the wires are unconnected, you can use a tone and probe (you can buy them at Home Depot) and take the probe unit all over the house to trace where the wire is going. This is tedious and time consuming, but it works fairly well.
3) If the contractor that ran the wires had their stuff together, the bundles will be behind the drywall and secured with a locator bundle. I don’t remember if it’s a powerful magnet, or some sort of RF unit, but that is what they used on my home. I nearly had a hissy fit when the drywallers covered up all of my wiring, and then the AV contractor came in with a probe/meter, and marked the locations of the wire bundles within a 1/2 inch! Pretty cool stuff!