The solution is to run both ethernet and phone over the same existing cat-5 cable. Every wall jack becomes two jacks, one RJ-11 for phone and one RJ-45 for ethernet. This neat hack could save you a lot of money, as you only have to buy new wall plates and jacks rather than wall plates, jacks, and hundreds of feet of wire.
See how this works in the next step.
Disclaimer: I'm not sure if this is legal. The telephone company won't be pleased if you short your telephone wires together. However, if you do everything right, they won't care. Don't blame me if you shock yourself (unlikely), damage Ethernet devices (also unlikely), damage phones (not as unlikely), damage your house wiring (not too unlikely), or damage your fingers with knives (rather likely).