Tim Fischer wrote:
As was mentioned, you need to plug a phone directly into the demarc box (the
place, usually outside, where the phone company's responsibility ends and
yours begins. There will be a modular jack here -- sometimes there's a
little plug that plugs there that connects all your house phone wiring,
other times its a hinged door. In any case, unplug this, and you plug a
phone in its place. If you get a good dial tone and ring, the phone company
is doing their job, and you have an interior wiring problem. If not, stop,
and call the phone company (actually, might be best to repeat the test with
a 2nd phone, just in case it happened to be that phone's fault).

If it's your problem, unplug every phone and then try just the phone you
tried the above test in, in a single jack. If it works, then you can plug
stuff in one at a time until you find the culprit. If it's still bad, you
have a more serious wiring issue -- possibly a minor short somewhere.

-Tim
..
While agreeing with the foregoing suggestions about how to trouble
shoot whether the fault is outside (telco) or inside (householder
wiring); a suggestion.
Could be bad jacks? Those small jacks with very little spacing between
the wire spring contacts can build up corrosion and 'tracking' between
the two little centre wires which are often the phone line pair. This
happens especially in a damp climate and/or the jacks are installed in
cool outside walls where warm house air can deposit moisture on them.
They are virtually impossible to clean. So replacement is best.
To trouble shoot you would disconnect them one at a time until the
trouble clears. Fix the trouble and then reconnect again one at a time
(could be more than one faulty).
Good luck.
PS. Back before we had competition the telco (monopoly) took
responsibility for the whole works and was concerned about your service.