Keep in mind, all modems are not created equal. Your ISP's equipment is top of the line, has a digital connection to the public telephone network, and is maintained well. The most obvious cause is that you've neglected to plug your telephone line into your modem. Double check to make sure. A common problem is plugging your telephone line into the wrong jack on your modem.
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-hook. These unexpected sounds can make the modem think there is no dial tone. Increase the amount of time that the modem waits for dial tone by adding commas before the number to be dialed. Check your voice mail and empty your mailbox so that the dialtone returns to normal.