On Thursday, May 29, 2014 7:14:24 PM UTC-7, micky wrote:
I have DSL, which of course uses standard copper phone lines. (All 4
wires, or just the red and green?)

Phone line out, DSL line in. Phone is always busy to callers from
outside. No dial tone for me, just silence.

When this happened to me, it was traced to a short-circuit between my house and the SLC terminal. One symptom was that the on-hook voltage was about 3V, which was attributed to corrosion between the splices of the wire pair (underground, about 10m from my house). There was enough signal getting through for my DSL to work, but the system thought my telephone was off-hook.