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