are two modem outlets on the back of your computer.
The correct outlet will be labeled as Line or will
have a picture of a wall jack next to it. Please verify
that the telephone cord is connected from the wall
jack directly into the correct modem outlet.
If you have other devices connected between the
wall jack and the modem (such as fax machines, caller
id boxes, or other phones), then these could be
interrupting the dial tone. Try disconnecting all
other devices - this may resolve the problem. You
can then connect the devices to a different wall