Can you tell me which version of tiger you are running - - the latest
is 10.4.6. I have seen known problems with the dial tone
dissapearing on 10.4.2, but the latest updates "should" fix that
problem.
Have you tried dialing with out waiting for the dial tone and seeing if it will work?
Open System Preferences > Click Network > Double-Click Internal
Modem > Click the Modem tab > uncheck the box that says "wait for
dial tone before dialing"
See if that will allow you to get online.
Something else to try :
change the phone cord your using.
plug into a different phone jack. I know you said that you
checked, just want to make sure that the cable you are using for the
computer isn't bad.
If these things don't solve the problem, then the modem itself may have gone bad.
Bottom line, unfortunately for the end user, Apple has a tendency to be
only forward looking. This can be good and it can be bad.
Since Modems are rarely used with the advent of cheap high speed
internet access, the software support of the modems is faltering.
In fact, the latest macs do not even come with modems anymore.
I wuld definitely run the Hardware Test disk and make sure that the modem is functioning properly.
Hope this helps you.