Here are some of the most common causes, and some solutions. When you pick up the phone, instead of having a clear dialtone, you have a series of broken dialtones that lasts for about 2 seconds. This stutter tone is enough to confuse the computer into thinking that there is no dialtone on the line, and so it just gives up and returns an error that it could not detect a dialtone. The solution to this is to check your phone messages, and then try to go back online. This will clear the stutter tone, thereby making it possible for the computer to dial out.
Lightning storms can damage computer modems, even without a direct lightning strike.
A simple static discharge on the phone line can fry a modem, leaving the computer itself intact. For this reason, we always recommend that if there is a thunderstorm looming on the horizon to unplug not only the computer from the power, but also to unplug the phone cord from the computer . This way, there is no way that lightning can get to your computer.