/diy/ - Do It Yourself

The house phones all stopped working normally. The digital phone blinks a light that lays line in use. I unplugged them all, looked at the modem and it kept blinking on the telephone light.
I can plug a phone into the modem directly and it works, so I assume the wire normally coming out of the modem leads to some splitters which connect to the other rooms. How can I test further to locate and fix the issue?