My small office has a DCS Compact with 5 terminals and 4 incoming lines. Today it started doing something odd: Line 4 (designated 704) stopped working. Picking it up from any terminal displays "Out of Service". I checked the incoming line by plugging it into a normal wireless phone we keep in a drawer, and there's dial tone on the line. What's more, if I swap the lines around (replug the line going to Line 3 into Line 4 instead), it's always Line 4 that gives the problem. The other 3 always have dial tone.

I'm thinking this is a hardware issue somewhere in the wires or the brain itself, but where do I start in checking that?