If you have phone company voice mail, you have to clear your box before trying to connect with a modem because of the message waiting signal. Also make sure you have the phone cord plugged into the right jack on the modem. Some switch between the jacks with a relay so if the phone cord is plugged into the wrong jack, it will never hear the dial tone.
. If you are sure it's plugged in correctly and you don't have telco VM, then it's new modem time. Inability to detect dial tone is a common failure mode for internal modems.
. Luckily the best are cheap. Lucent/Agere chipset generics are less than $15. And a decent external can be had for under $35. (CenDyne at Provantage) if you want the most flexibility.
. Once you have your new modem, be sure to protect it and your system with surge protection and/or unplug from the wall when not using. I worked at a service shop in FL (lightning capital of the world) for awhile and we used to make big bucks every spring and fall replacing modems and repairing systems of the clueless.
.bh.