First, check that your ringer is on and the volume is high enough.
If you are subscribed to Call Forwarding, it might be activated. Dial *73 to deactivate it.
If you only have one phone line, check that the phone wire is plugged into port 1 on the back of the modem, not port 2. If you have two lines, check that each wire is plugged into its respective port.
If your phone still does not ring, unplug all of your phones from their wall jacks and test them one at a time. To do this, plug one phone back into the wall jack and check for a dial tone. If there is still no dial tone, unplug this phone and try with another phone.