I get that error message on occasion when I'm traveling and have changed my Pismo settings from at-home DSL to on-the-road dial-up.
The first thing I check is whether another phone on the circuit is in use, hence (and in actuality) "no" dial tone.
2d: I triple-check the seating of the line cord, both in its wall jack connection and in the port on the Mac.
Awfully simple, I know, but those steps have fixed a lot of what initially appeared to be serious problems