I bought a modem recently which can not find a dial tone. I have run tests on the computer which appear to show its working fine.
I have both the phone and the internet cable going into the one phone socket, and when the internet cable is connected to the modem, I get no dial tone from the phone (even if the computer is off!)
I have tested the connection on an other computer and it worked, so it's not the connection that's the problem.
And the reason I bought the modem was because I had the same problem with the previous modem.