This error usually just means that someone else is talking on the line when you attempt to use it with your modem. You should be able to determine if this is the case by picking up a handset on the same phone line. If you hear a clear dial tone, you should then check the physical connections to the modem. If you hear a stutter tone, you will need to remove any waiting voice mail messages before your modem will be able to dial.  Be sure that the telephone cord is connected straight from the wall jack directly into the modem's outlet labeled Line or Telco or has a picture of the wall jack on it. Do not plug it into an outlet labeled Phone, or and outlet that has a picture of a handset on it.  Also, remove any other devices that may be in between the wall jack and the modem (Examples: splitters, surge protectors, or a fax machine).  If still unable to receive a dial tone you would want to have the modem tested to determine of the modem can still "hear" a dial tone.