If you have call answer service on your line and have messages waiting, the modem will not recognize this as a dial tone. You will need to empty your message box before dialing out with your modem. The most obvious cause is that you've neglected to plug your telephone line into your modem. Double check to make sure. Additionally, the telephone cable may be bad, test it on a regular telephone.
Many modems have two jacks, one for the telephone line and one for a telephone handset.
Double check to make sure. Double check to make sure that all other devices are on the hook. These unexpected sounds can make the modem think there is no dial tone. To show the actual connection speed, add W2 to your initialization string. It takes a few seconds to do this retraining, and these are the pauses that you may experience.